Tektronix 300 Instruction Manual
INSTRUCTION MANUAL
MODEL 300
ELECTROMETER
OPERATIONAL AMPLIFIER
MODEL 300 OPERATIONAL AMPLIFIER
TABLE OF CONTENTS
TABLE OF CONTENTS
Section
1. GENERAL DESCRIPTION . . . . . . . . 1
l-l.
l-2.
l-3.
l-4.
1-5.
l-6. Equipment Shipped . . . ,. . . 3
2. OPERATION . . . . . . . . . . . . . 5
2-l. Terminals . . . . . . . . . . 5
2-2.
2-3. power supply . . . . . . . . 6
2-4.
2-5. Output Connections . . . . . 8
2-6.
2-7.
2-E.
2-9. Current Integrator or
2-10. Impedance Matching Amplifier 15
2-11. Voltage Amplifier . . . . . . 17
2-12. Other Modes of Operation . . 18
2-13. Stability, Frequency Response,
Z-14. Remote Zero Control . . . . . 19
2-15. Connections and Use of
General . . . . . . . . . . . 1
Features . , . . . . . . . . 1
Specifications . , . . + . . 2
Operating Modes : , . . , . . 3
Accessories . . . . . . I . . 3
Mounting Instructions . . . 5
Input Connections . . . , . . 7
Modes of Operation . . . . . 8
Linear Current Amplifier . . 10
Logarithmic Current Amplifier 12
Charge Amplifier . . . . . . 14
Andy Oscillation . . . . . . 18
Reference Voltages . . . . . 20
Page
Section
MAINTENANCE . . . . . . . . . . . 23
4.
4-l.
4-2.
4-3.
4-4. Troubleshooting . . . . . . . 24
4-5.
4-6.
4-7.
ACCESSORIES . . . . . . . . . . 29
5.
5-l.
5-2.
6.
REPLACEABLE PARTS . . . . . . . . . 35
6-l. Replaceable Parts List . . . 35
6-2. How to Order Parts . . . . . 35
General . . . . . . , . . 23
Maintenance Schedule . . . . 23
Parts Replacement . . . . . 23
Troubleshooting Positive and
Negative Power Supply
Regulators . . . . . . . . . 25
Troubleshooting the Amplifier 26
Calibration . . . . . . . . . 26
Model 3012 Power Supply . . , 29
Model 3011 Shielded Switch . 31
Model 300 Replaceable
Parts List . . . . . . :. 36
Model 3012 Replaceable
Parts List . . . . . . . . 38
Model 300 Schematic
Diagram 195581) . . , . . . 4.1
Model 3012 Schematic
Diagram 20351B . . . . . . 43
Page
CIRCUIT DESCRIPTION . . . . . . . . 21
3.
3-l. General . . . . . . . . . . . 21
3-2. Electrometer Input . . . . . 21
3-3. &lid State
Differential Amplifier . . . 21
3-4.
3-5.
0167R
Positive Pkvar Supply
Regulator . . . . . . . . . 22
Negative Power Supply
Regulator . . . . . . . . . 22
* Change Notice , . . . . . . . .Last Page
* Yellow Change Notice Sheet is included
only for instrument modifications
affecting the Instruction Manual.
i
MODEL 300 OPERATIONAL AMPLIFIER
GENERAL DESCRIPTION
GENERAL DESCRIPTION
sing&-end-
lo-
l-l.
GENERAL.
a.
The Keithley Model 300 Electrometer Operational Amplifier is a compact,
ed amplifier.
SECTION 1.
It has greater than 1014 ohm input resistance and less than 5 x
ampere current offset. Its current drift is less than lo-l5 ampere/24 hours and its current noise is less than 5 x LO-l5 ampere peak-to-peak.
Primarily a current amplifier, the
Model 300 will operate with signals from LO-l4 to 10T2 ampere.
b.
Open
C.
The Model 300 is completely solid state except for the input stage.
input tubes are used because they have less noise, better
loop
dc voltage gain is 20,000; output is fll volts, 11 milliamperes.
stability,
Electrometer
and are less sensi-
tive to voltage transients than other high impedance devices now available.
l-2. FEATURES.
a. The Model 300 will operate from inexpensive power supplies or batteries.
ted supplies with outputs from +16
to
125 volts and -16 to -25 volts will power the Model
Unregula-
300. The Amplifier Will also operate from standard 15-volt regulated supplies.
An internal zero control is built into the Model 300.
b.
In addition, a potentiometer
can be connected externally to allow remote zeroing.
c. A regulated reference or polarizing potential of ~13.5 volts at 1 milliampere is
available.
d. Electrostatic and electromagnetic
shields are provided to insure minimum pick-
Therefore, input and feedback elements
UP.
can be mounted within the Model 300 case
with complete shielding.
e.
The Model 300 withstands input over-
loads up to 2400 volts and will not be dam-
aged by induced static voltages with the
input open.
f.
An internal compensation network
greatly reduces the possibility of oscil-
lation regardless of external circuitry.
The Amplifier is constructed on a
g.
3-l/2 inch by 4 inch prip.ted circuit board
that forms a 15-terminal card-edge connector.
coaxial connector,
The input is a Teflon-insulated
Space and mounting ter-
minals are provided for internally mounting
input,
dances.
feedback and output dividing impe-
FIGURE 1. Keithley Instruments Model 300
Electrometer Operational Amplifier.
0566R 1
GENERAL DESCRIPTION
MODEL 300 OPERATIONAL AMPLIFIER
l-3.
DC VOLTAGE GAIN,, OPEN LOOP:
INPUT CHARACTERISTICS:
FREQUENCY CHARACTERISTICS:
SPECIFICATIONS (Measured at 250C).
Unloaded:
lOOO-ohm load: Greater than 12,000.
Resistance:
Capacitance: Less than 10 picofarads.
Current Offset:
Drift: Less than lo-l5 ampere/24 hours.
Temperature Coefficient: Less than LO-15 ampere/OC.
Voltage Offset: Adjustable to zero.
Drift: Less than 500 microvolts/hour averaged over any 24-hour period after two-'
hour warm-up.*
Temperature Coefficient:
Input Voltage Noise:
(0.1-10 cps): Less than 5 microvolts rms.
(10 cps-100 kc): Less than 5 millivolts rms.
Current Noise (0.1-10 cps): Less than 5 x LO-15 ampere peak-to-peak.
Overload Limit: ?400 volts.**
Closed Loop Unity Gain, Small Signal: dc to 100 kc (-3db).
Slewing Rate: l'voltfmicrosecond minimum.
Gain Bandwidth Product: Greater than 150 kc.
Rolloff:
Greater than 20,000.
Greater than 1014 ohms.
Less than 5 x LO-l4 ampere.
Less than 500 microvoltsf°C.*
Approximately 6 db/octave.
OUTPUT:
Amplifier: rll volts at 11 milliamperes.
Reference Voltage:
input.
OPERATING TEMPERATURE: 0 to 5O'C.
CONNECTORS: Input: push-on coaxial receptacle, Amphenol 2175. All other connections:
15-terminal l/16 inch card-edge.
POWER REQUIREMENTS:
+16,to +25 volts unregulated, 35 milliamperes plus output current;
-16 to -25 volts unregulated,
Note: Model 300 will also operate to specifications with standard 15-volt fO.l%
regulated power supplies.
DIMENSIONS, WEIGHT:
13 ounces.
ACCESSORIES SUPPLIED: Mating card-edge connector and Teflon-insulated coaxial input
connector with shield (chassis mounting).
*With 100% feedback this drift as a percent of full output is less than O.O05%/hour (or/OC,)
*with a LO5 ohm or greater feedback resistor without a shunting capacitor.
several hours to recover to specified drift with severe overload.
+13.5 volts at 1 milliampere, regulated to *O.l% for 10% change in
8 milliamperes plus output current.
3-l/2 inches high x 4 inches wide x l-1/2 inches deep; net weight,
May require
2
0466R
MODEL 300 OPERATIONAL AMPLIFIER GENERAL DESCRIPTION
l-4.
veniently used in a number of operating modes:
current amplifier, current integrator and charge amplifier.
OPERATING MODES. The Model 300 is primarily a current amplifier.
linear current amplifier, logarithmic
Section 2 describes these and
It can be con-
other operating modes for the Model 300.
1-5. ACCESSORIES (See Section 5.)
Model 3011 Sheilded Switch can be used with the Model 300 where range switching is
a.
required. The Switch is a 3-pole, &position, adjustable stop switch. The Model 3011 is
constructed for low leakage and to provide shielding for the components.
Refer to Section
5 for complete description.
b. Model 3012 Power Supply is designed to power 1,2 or 3 Model 300s.
The Model 3012
delivers positive and negative outputs between 16 and 25 volts, which fill all the power
requirements of'the Model 300. The Power Supply can be floated up
to
500 volts off
chassis ground.
High megohm resistors are available for using'in the Model 300.
c.
These resistors may
be mounted internally within the Amplifier or in the Model 3011 Shielded Switch. The
values available are:
1. Model R20-109 High Megohm Resistor; log ohms +3%.
Model R20-lOlo High Megohm Resistor; 1010 ohms *3%
2.
Model R20-10'1 High Megohm Resistor; LOLL ohms ?3%.
3:
4. Model R20-lQl* High Megohm Resistor; 101' ohms ?3%.
Model R2O-10L3 High Megohm Resistor: 1013 ohms rlO%.
5.
1-6. EQUIPMENT SHIPPED.
open loop opefational amplifier.
Feedback and input resistors are not included.
The Model 300 is shipped factory calibrated and connected as an
The built-in zero potentiometer (R112) is connected.
Shipped with the Model 300 is a mating 15
terminal card-edge connector, and Teflon insulated input connector with shielding hood.
06678
3
OPERATION
MODEL 300 OPERATIONAL AMPLIFIER
Pin No.
Designation
INPUT INPUT
1
ALT. INPUT
2 GUARO
3 B
4
A
5 -REF
6 -16V to -25V
7 ---
8 OUTPUT
9
10
+REF
+16V to t25V
11 --12 ---
13
C
14 FEEDBACK
15 GROUND
Description Paragraph
Teflon Insulated Coaxial Input
May be used for large signals
Used with ALT. INPUT
For remote mounting zero control
For remdte mounting zero control
Negative Reference Voltage (-14 volt output)
Negative Voltage Supply Input
Not used
Output
Positive Reference Voltage (+13.5 volt output)
Positive Voltage Supply Input
Not used
Not used
For remote mounting zero control
For fractional feedback
Circuit Ground
TABLE 1. Model 300 Terminal Explanation.
Zero Adjust
(Rll2)
Reference
2-4
2-4
2-4
2-14
2-14
2-15
2-3
---
2-5
2-15
2-3
---
---
2-14
2-7
2-4
FIGURE 2.
terminals.
4
Model 300 Terminal Designations. Kerer to Table 1 tar explanation or the
0666R
MODEL 300 OPERATIONAL AMPLIFIER
~~~e~~~~~~~~~,i
OPERATION
SECTION 2.
2-l.
The 15-terminal card-edge is used for all other connections. The mating connector for
the 15-terminal card-edge is keyed to prevent improper insertion.
Table 1 for terminal identification and explanation.
2-2.
in a convenient location within a system.
Part No. CS-175-15) and shielded input mating connector (Keithley Part No. CS-179) with
the hooded shield (Keithley Part No. W-180) to a surface. Then plug the Model 300 into
the mating connectors. This mounting is acceptable for all positions except when the
mating connectors are above the Amplifier. Then, it may be necessary to use a bracket
or similar device to hold the Model 300.
TERMINALS.
MOUNTING INSTRUCTIONS.
a. The Model 300 is designed for use in various measurement systems. It easily mounts
b. To mount the Amplifier, attach the furnished card-edge mating connector (Keithley
c. Mounting the mating connectors CS-179 and CS-175-15. (See Figure 3 for dimensions.)
1. Select the desired surface for mounting.
The Model 300 input connector is a Teflon-insulated coaxial receptacle.
OPERATION
Refer to Figure 2 and
' 2. Drill two l/B-inch diameter holes 2-15/16 inches apart to mount the mating 15-
terminal card-edge connector.
3. Drill the 21/64,-inch diameter hole for the mating coaxial
to Figure 3 for exact positions.)
4,. Cut out 2-7116 inch x 3/B-inch plot in chassis.
r-
3-1/411
-
input connector.
(Refer
I
Mating Connector Outline
21/64" Dia.
for Coaxial Connector
FIGURE 3. Mounting Hole Diagram.
Keithley Part Nos. CS-175-15 and CS-179.
0167R
This gives dimensions for mounting
mating connectors,
5
OPERATION
Fasten the connectors in their proper positions.
5.
Attach the Model 300 to its mating connectors.
6.
The terminals of the mating connectors correspond exactly to,the terminals of the
d.
Model 300 shown in Figure 2.
and CS-175-15 is the mating connector for the card-edge connector.
2-3. POWER SUPPLY.
a. The instantaneous value of the supply voltages must be between 16 and 25 volts.
For example, even a power supply whose value varies
only exception is that a power supply with as low as a 15-volt output regulated to 0.1%
can be used. Connect the positive supply
to the +16V to +25V Terminal (pin 10,
Figure 2).
to the -16V
ure 2).
The Model 300 can operate from unre-
b.
gulated power supplies whose minimum instantaneous output falls below 16 volts.
However, the positive reference output must
be set below the normal 13.5 volts by ad-
justing the Regulator Adjust Potentiometer,
R209, (Figure 22). For example, to use a
power supply with a 15-volt minimum output,
set the positive reference output to 12.5
volts. Reducing the 13.5 volts of the
positive reference output reduces the max-
imum Amplifier output voltage by a 1:l
ratio; that is, reducing the reference
output one volt reduces the maximum output
voltage one volt.
R209 is not necessary for 15-volt supplies
with 0.1% regulation.
The graph in Figure 4. shows where the
C.
Model 300 operates satisfactorily. Re-
liable operation is obtained wherever the
slope of the positive reference output
voltage versus the supply voltage is approx-
imately 2 millivolts per volt or less.
Reliable operation is also obtained wherever
the slope of the negative reference voltage
output versus the supply voltage is approx-
imately 20 millivolts per volt or less.
The Keithley Model 3012 Power Supply
d.
provides all the power necessary to drive
up to 3 Model 300s (See Section 5). Or,
a power supply can be constructed using
the circuit shown in Figure 5.
Connect the negative supply
to
-25V Terminal (pin 6, Fig-
Adjusting potentiometer
CS-179 is the mating connector for the INPUT Receptacle,
from
FIGURE 4,.
Voltages for the Model 300. The Model 300
will operate satisfactorily for a given
supply voltage and reference output as long
as the slope of the curve is approximately
2 millivolts/volt. As long as the voltage
is between 16 and 25 volts, no adjustment
is necessary. For instantaneous voltages
below 16 volts,
ence voltage to remain within the zone of
reliable operation.
potentiometer R209,
output,
gulated power supply whose instantaneous
voltage falls to 15 volts.
apply for the negative supply voltage. The
only variation is that the slope is approx-
imately 20 millivolts/volt.
MODEL 300 OPERATIONAL AMPLIFIER
16 to 25 volts can be used.
Allowable Unregulated Supply
reduce the positive refer-
For example, adjust
the positive reference
to 12.5 volts when using an unre-
Similar curves
The
?
6
0167R
MODEL 300 OPERATIONAL AMPLIFIER
e. Change the positive reference output
by measuring the output at pin 9 with the
Keithley Model 153 and adjusting potentiometer R209 (Figure 22).
2-4.. INPUT CONNECTIONS.
a. Normally all input signals should be
through the INPUT Receptacle.
It is specially insulated and shielded to minimize
noise which will distort the input signal.
b. For high impedance measurements,
carefully shield the input connection and
the source being measured, since power line
frequencies are well within the pass band
of the Amplifier. Unless the shielding is
thorough, any alteration in the electrostatic field near the input circuitry will
cause extraneous signals to appear at the
output.
OPERATION
:
Circuit of a Power Supply for the
Model 300. Keithley Part No. TR-78 can be
used as the transformer in the circuit.
c. Use high resistance, low-loss materials - such as Teflon (recommended) and polyethylene - for insulation. The insulation leakage resistance of test fixtures and leads
should be several orders magnitude higher than the internal resistance of the source. If
it is not, leakage losses will cause inaccurate readings. Coaxial cables used should be
a low-noise type which employ a graphite or other, conductive coating between the dielectric and the surrounding shield braid.
Amphenol-Borg Electronics Corporation, Microdot,
Inc., and Simplex Wire and Cable Company make satisfactory types.
NOTE
Clean and dry connections and cables are very important to maintain the value
of all insulation materials. Use pure CH30H methyl alcohol to clean Teflon
insulation.
d. Any change in the capacitance of the measuring circuit to ground will cause extraneous disturbances.
ing cables to prevent their movement.
at the output as a signal;
Make the measuring setup as rigid as possible, and tie'down connect-
If a continuous vibration is present, it may appear
other precautions may be necessary to isolate the instrument
and the connecting cable from the vibration.
NOTE
Unless otherwise specified all instructions in this manual are for input signals
through the INPUT Receptacle.
e. If it .is more convenient,
larger input signals.
come less effective.
Note that input leakage current may increase and shielding will be-
However,
the ALT. INPUT Terminal (pin 1, Figure 2) can be used for
the input tape will still remain guarded.
the INPUT Receptacle to the card-edge ALT. INPUT:
Remove jumper Q-R (Figure 6).
1.
0167R
To change from
7
OPERATION
Connect terminal R to hole P (Figure 6) located,at the end of the ALT. INPUT tape.
2.
NOTE
Do not use the GUARD Terminal for GROUND, even though GUARD and GROUND are shown
connected in the schematic diagram.
The Model 300's input overload limit of f400 volts is for LO5 ohms or greater feed-
f.
back impedance.
feedback currents.
feedback impedance is acceptable.
pendent upon frequency. As frequency increases, impedance is reduced:
This ensures that the output stage will not be damaged by large transient
Any combination of resistance and capacitance with 105 ohms minimum
However, when using a capacitor, the impedance is de-
Severe ground loops may result.
MODEL 300 OPERATIONAL AMPLIFIER
1
zc = 2nfC
where ZC is the impedance in ohms;
f is the frequency in cps;
c is the capacitance in farads.
equation 1
Therefore,
greatly and the impedance decreases.
than 105 ohms.
2-5. OUTPUT CONNECTIONS.
a. The output voltage is through the OUTPUT Terminal (pin 8, Figure 2). Almost any
means of looking at the output voltage can be employed as long as the load on the output
is not less than 1000 ohms.
Excessive capacitance loading at the output (usually greater than 1000 picofarads) will cause oscillation.
pensation elsewhere in the circuit (paragraph 2-13).
Output overload protection is provided for the Model 300. Thus a temporary direct
b.
short to ground at the output is harmless.
with the Model 300 in a saturated or near saturated state, may damage the Amplifier.
2-6.
through simple adjustment of its circuitry. Paragraphs 2-7 through 2-12 describe the con-
struction and use of several of the modes.
each operating mode.
MODES OF OPERATION.
The Model 300 Operational Amplifier can be used in many different modes of operation
a.
if spikes or steps are present in the overload signal, the frequency increases
In this situation the feedback impedance is less
This can usually be stabilized through com-
However, extended periods of shorted output,
Refer to Table 2 for the paragraph describing
The Model 300 can be used in these different operating modes through the mounting
b.
of various elements in the feedback and input circuits.
within the Model 300 case with complete shielding. To connect multiple feedback or input
elements, use the Model 301lShielded Switch. The mode of operation is determined by the
type of component used and where it is placed in the circuit.
8
The components can be mounted
0666R
.~
MODEL 300 OPERATIONAL AMPLIFIER
OPERATION
FIGURE 6.
Within Model 300.
Circuit Points
Points are
used to construct various circuits referred to in Figures
8 through 15.
aegulaCar
I
H-
Ad,“It J
FIGURE 7.
Circuit Points
Within Model 300. Points ax
used to construct various
circuits referred to in Fig-
ures 8 through 15.
0666
E-
K-
OPERATION
MODEL 300 OPERATIONAL AMPLIFIER
Operating Mode
Linear Current Amplifier Without Fractional Feedback
Linear Current Amplifier With Fractional Feedback
Logarithmic Current Amplifier
Current Integrator or Charge Amplifier
Impedance Matching Amplifier 2-10
Voltage Amplifier Without Fractional Feedback 2-11
Voltage Amplifier With Fractional Feedback 2-11
Other Circuits 2-12
TABLE 2.
c.
minals in these figures refer to the lettered points in Figures 6 and 7.
figures to connect jumpers and to mount elements.
d.
voltage for no input signal. There is a hole in the cover (Figure 2) enabling this adjustment to be made without removing the cover. Refer to paragraph 2-14 for other adjustments.
2-7. LINEAR CURRENT AMPLIFIER (Figures 8 and 9).
a.
amplifier.
input current.
Model 300 Operating Modes and Paragraph Describing the Modes.
Figures 8 through 15 illustrate the various modes of operation.
Adjusting the Zero Adjust Potentiometer, R112 (Figure 7), sets the output to zero
Placing a resistor in the feedback loop converts the Model 300 to a linear current
The output voltage depends on the magnitude of the feedback resistor and the
Paragraph
2-7
2-7
2-8
2-9
The lettered ter-
Ref.er to the
"out = -Iink
where Vout is the output voltage in volts;
Ii,, is the input current in amperes;
Rfb is the feedback resistance in ohms.
For a given iriput current the output voltage can be chosen by selecting the, feedback resistor, Rfb,
resistor accessories), V,ut can be as much as fll volts.
sible results in a better signal-to-noise ratio at the output.
Using fractional feedback increases the output voltage gain although drift and noise
b.
also increase. Fractional feedback is useful for amplifying different current levels
while using only one high megohm resistor in the feedback loop.
Be careful when handling the high megohm resistors,
the ends of the leads; do not touch the glass.
resistor value,
Connections for a linear current amplifier without a fractional feedback (Figure 8):
=.
1. Connect ,jumpers across terminals Q-R, R-S,
By using the largest possible feedback resistor (such as the high megohm
Keeping V,,t as large as pos-
Hold these resistors by
Contamination will change the
S-D, E-H, and H-J (Figures 6 and 7)
equation 2
10
0666R
MODEL 300 OPERATIONAL AMPLIFIER
OPERATION
FIGURE 8.
out Fractional Feedback.
The Model 300 is shipped from the factory in the linear current amplifier mode
minus the element in the feedback loop.
connected across terminals Q-R, R-S, S-D, E-H and H-J.
2. Remove any element of jumper from across E-K (Figure 7).
3. Mount the feedback resistor, Rfb,
4. The output voltage for the Model 30.0 as a linear current amplifier without a
fractional feedback is given by equation 2.
d. Connections for a linear current amplifier with a fractional feedback (Figure 9):
1.
2. Mount the feedback resistor, Rfb,
3. Mount fractional resistors Rl
respectively.
Linear Current Amplifier With-
---
Connect jumpers across terminals Q-R, R-S, S-D and E-H (Figures 6 and 7).
and R2 (Figure 9) across terminals J-H and H-K
FIGURE 9. Linear Current Amplifier With
FractionaL Feedback.
NOTE
Therefore it is shipped with jumpers
in the feedback loop across terminals D and E.
--
in the feedback loop across terminals D and E
4. The output voltage for the Model 300 as a linear current ampiifier with fractional
feedback is
v
= -IinRfb
out
where V,ut is the output voltage in volts;
Iin is the.input current in amperes;
Rfb is the feedback resistance in ohms;
Rl and R2 are the divider resistances in ohms.
NOTE
The current through fractional resistors R1
that through the feedback resistor, Rfb,
R1 + R2 should be greater than 1 kilohm so as not to overload the output.
0766R
and R2 should be much greater than
to maintain proper amplification. Also,
equation 3
11
OPERATION
Feedback
Resistor
1olOn
1olCn
MODEL 300 OPERATIONAL AMPLIFIER
1012Sl
101*sl
% Feedback
Output Voltage
Input current
Resolution
100%
10%
10 v 10 v
10-9 amp 10-10 amp
10-11 imp 10-12 amp
100%
10 v
10-11 amp
10-13
amp
10%
10 v
10-12
amp
10-14 amp
Current Offset,
% of output
0.005%
0.05%
0.5%
5%
Drift/Hour
% of output
Ovserved Rise Time
,
0.005%
10 msec
0.05% 0.005%
20 msec
200 msec
0.05%
300 msec
TABLE 3. Typical Performance Values for the Model 300 Used as a Linear Current Amplifier.
Offset, drift and rise time are affected by the circuit used, but the above table shows
some of the Model 300's capabilities.
3;
100% is
e.
with
no fractional feedback.
To mount the divider outside the Model 300, do the following:
---
1. Connect jumpers across terminals Q-R, R-S,
"% Feedback" refers to fractional feedback equation
--- -S-D and E-H (Figures 6 and 7).
2.
Remove all elements from.J-H and H-K.
3. Mount the feedback resistor, Rfb,
in the feedback loop across terminals D and E.
Connect FEEDBACK (pin 14, Figure 2) or terminal H to the center of an external
dizider. (T
5. Connect one end of the divider to OUTPUT (pin 8, Figure 2) or terminal J.
minal 3 is connected to OUTPUT).
2-a.
LOGARITHMIC CURRENT AMPLIFIER (Figure 10).
erminal H is connected to FEEDBACK).
(Ter-
(See NOTE paragraph 2-7d).
a. Silicon diodes or transistors in the feedback loop make the Model 300 a logarithmic
current amplifier.
The log characteristic of the element used determines the amplifier
performance. lN459 diodes and silicon transistors usually provide 7 to 9 accurate decades.
The leakage current of the element should be at least two magnitudes less than the current
being measured.
b. The circuit for the logarithmic current amplifier is shown in Figure 10.
cuit uses a single diode and is for positive currents.
V
= -A log Iin
out
The output voltage is:
This cir-
equation 4
where Vout is the output voltage in volts;
Iin is the input current in amperes;
A is a positive constant, dependent upon the characteristic of the diode.
12
0766R
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