Transistor biased amplifier has a biased diode discriminator driven by a high impedance /several megohms/ current source, rather than a voltage source with several hundred ohms output impedance. This high impedance input arrangement makes the incremental impedance of the threshold diode negligible relative to the input impedance

Larsen, R. N.

English

NASA Technical Reports Server

Bias 
Current 
Generator -
August 1967
	 Brief 67-10311 
•
1GY C
AEC-NASA TECH BRIEF 
FES
AEC-NASA Tech Briefs describe innovations resulting from the research and development program 
of the U.S. AEC or from AEC-NASA interagency efforts. They are issued to encourage commercial 
application. Tech Briefs are published by NASA and may be purchased, at 15 cents each, from 
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Transistor Biased Amplifier Minimizes Diode Discriminator 
Threshold Attenuation 
Voltage to	 Diode 
Current	 Discriminator Post
	
Stretcher	 Complementary 
Converter	 Amplifier	 Emitter 
Bias Input	 Level Control 
The problem: 
Diode nonlinearity in biased amplifiers causes 
considerable signal attenuation above the diode 
threshold level. When the threshold diode begins to 
conduct, the diode impedance is not negligible relative 
to the source impedance, resulting in nonlinear signal 
attenuation near the diode threshold. 
The solution: 
A transistor biased amplifier in which the biased 
diode discriminator is driven by a high impedance 
(several megohms) current source, rather than a 
voltage source with several hundred ohms output 
impedance. This high impedance input arrangement 
makes the incremental impedance of the threshold 
diode negligible relative to the input impedance. 
In this configuration, the output current (and voltage) 
of the amplifier is a linear function of the input current
over a large dynamic range, quite independent of the 
threshold diode impedance. 
How it's done: 
The input voltage pulses are converted to current 
pulses by the voltage-to-current converter circuit, 
which consists of an operational amplifier with a 
Darlington Pair output. The current pulses are 
applied to the diode discriminator circuit, consisting 
of Di and D2. Diode Di quiescently conducts a bias 
current which is adjustable. D2 is quiescently non-
conducting, until the input signal exceeds the thresh-
old bias at Di. At this time, Di is completely cut off 
and the signal, in excess of the threshold level, flows 
through D2 into the post amplifier. The post amplifier 
consists of an operational amplifier whose input is 
essentially grounded (grounded base input amplifier). 
The post amplifier converts the excess current signal 
(continued overleaf) 
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https://ntrs.nasa.gov/search.jsp?R=19670000311 2020-03-11T20:41:16+00:00Z
to a voltage signal. The open loop gain of the ampli-
fier is kept high due to a bootstrap amplifier configu-
ration. The gain of the amplifier circuit can be ad-
justed by changing the value of Rf. For example: for 
a gain of 10, Rf=10 Kohms; for a gain of 5, Rf=5 
Kohms, etc. This stage is designed to provide a fast 
rise time to prevent pulse amplitude distortion at high 
bias levels in the system. The output of the post ampli-
fier is fed through a stretcher circuit to reproduce the 
original shape of the voltage pulse at the input to the 
voltage-to-current converter. This stretcher circuit is 
not required if the input pulses are square wave rather 
than R-C clipped. The output of the stretcher circuit 
is tied to a complementary emitter follower, which is 
used effectively as a cable driver. 
The threshold bias is produced by the bias current 
generator, which consists of two operational ampli-
fiers and two feedback amplifiers. The bias reference is 
variable from ground to - 10 volts. The reference to 
the bias current generator can be supplied either 
internally or externally (in which case a ramp wave-
form generator can be used). Internally the reference 
window is variable by the window level control. The 
output of the bias current generator is tied to the diode 
discriminator circuit and controls the threshold level 
of the system. 
Notes: 
1. Selection of the diodes (Di and 132) for the diode 
discriminator is not necessary,. since the conduc-
tance characteristics of the diodes have no effect on 
the basic operation of the circuit.
2. This circuit can also be used as a linear gate by 
applying a fixed current pulse rather than variable 
DC current to the discriminator diode Di. 
3. Reference: Nuclear lnstrunieiits and Methods 32, 
(1965), p. 147-151. 
4. Inquiries concerning this innovation may be di-
rected to:
Office of Industrial Cooperation 
Argonne National Laboratory 
9700 South Cass Avenue 
Argonne, Illinois 60439 
Reference: B67-10311 
Source: R. N. Larsen—Electronics Division 
(ARG- 163) 
Patent status: 
Inquiries about obtaining rights for commercial 
use of this innovation may be made to: 
Mr. George H. Lee, Chief 
Chicago Patent Group 
U : S. Atomic Energy Commission 
Chicago Operations Office 
9800 South Cass Avenue 
Chicago, Illinois 60439 
Brief 67-10311	 Category 01


Transistor biased amplifier minimizes diode discriminator threshold attenuation

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