Circuit limits slope of arbitrary waveform to avoid exceeding rate limit of subsequent amplifier, or to form trapezoidal wave with adjustable rise and fall rates from square wave of arbitrary frequency. Integrator provides delay needed to develop output waveform. DC coupling is used to preserve original dc offset

Engel, A.

English

NASA Technical Reports Server

	August 1973	 B7340346 
•	 NASA TECH BRIEF 
NASA Pasadena Office 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Operational Slope-Limiting Circuit 
R3	 CI 
A1 CR
	
iI>i	 .iI>__ 
	
 Input
	
Output 
The problem: 
To limit the slope of an arbitrary waveform in 
order to avoid exceeding the rate limit of a subse-
quent amplifier, or to form a trapezoidal wave with 
adjustable rise and fall rates from a square wave of 
arbitrary frequency. 
The solution: 
A circuit consisting of an amplifier, an inverter, 
and an integrator; the integrator provides the delay 
needed to develop the output waveform. DC coupling 
is used to preserve the original DC offset. 
How it's done: 
The circuit is shown in the diagram; the output of 
the first of three general-purpose operational ampli-
fiers is bridged to its input by a double-anode zener 
or two closely-matched back-to-back zeners (CR1) 
with breakdown voltages within the excursion capa-
bility of the amplifier (say, 8 volts). The amplification 
of the second amplifier (an inverter) is set approx-
imately at unity by resistors of the order of 10 kilohxns 
(± 5%). The input resistor to the last amplifier is set
at a few kilohms to 100 kilohms or more, and the 
integrating capacitor can be from 100 pf or less to 1 
mfd or more in order to set the slope limit. For ad: 
justable slope operation, the input resistor to the last 
stage may be variable and the capacitor value is 
altered by switching. The output resistor in the last 
stage is optional but, if used, it determines the circuit 
output impedance, and permits use of large capaci-
tances which otherwise must be limited to about 0.05 
mfd by the choice of the operational amplifier. The 
main feedback resistor, R6, and the input resistor, R1, 
determine the overall gain of the circuit; generally, 
they will be 'a closely-matched pair (say, 10 kilohms 
each). The series resistor-capacitor combination, R7 
and C2, across R6 compensates the loop gain which, 
for a small signal, is the product of the gains of the 
first and third amplifiers (typically about 200 dB); the 
RC combination also stabilizes the gain. The optimum 
value of the RC combination is determined empir-
ically, and will be governed by the characteristics of 
the operational amplifiers, but if the integrating cap-
acitor, Ci, is switched, then the RC compensating 
combination must also be switched.
(continued overleaf) 
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When the input signal changes rapidly, the output 
is prevented from following it by the delay in the 
integrator; consequently, a large error exists in the 
loop and the output of the first amplifier rises until it 
is clamped by CR1. A constant voltage is applied to 
the inverter, causing the integrator to slew at a uni-
form rate until the output very nearly equals the 
input. 
When the change of input signal is sufficiently slow 
to allow the integrator to keep up with it, the circuit 
operates linearly, exactly reproducing the input wave-
form. (Note that the output signal is inverted in po-
larity with respect to the input).
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP .73-10348 
Patent status: 
NASA has decided not to apply for a patent. 
Source: Alexander Engel of 
Caltech/JPL
under contract to
NASA Pasadena Office
(NPO-11773)
. 
[IJ 
B73-10346	 Category 01


Operational slope-limiting circuit

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