Circuit provides a pulse source of constant frequency with a duty cycle that is adjustable by an external input signal. It could serve as a switching mode voltage regulator or as a switching source for control systems

Johnson, J. E.

English

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+v 
External 
chronization 
if Desired 
October 1969	 Brief 69-10512 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Constant-Frequency, Variable-Duty-Cycle Multivibrator 
The problem: 
To provide a pulse source of constant frequency 
with a duty cycle that is adjustable by an external 
input signal. Such a circuit would be most useful as a 
switching mode voltage regulator and could also find 
use as a switching source for a variety of control 
systems. 
The solution: 
A circuit in which the above requirements are met 
and which may easily be synchronized by an external 
signal without interfering with the operation of the 
duty cycle control.
How it's done: 
Assume transistor T 2 is on. The capacitor C 1 has 
previously charged to potential V i . With T 2 on, ca-
pacitor C 1 supplies a voltage -V 1
 to the base of T 1, 
'thUs holding it off. The èurrent source formed by tran-
sistor T 3 and its bias resistors R 31, R 32, R 3 3, causes 
this potential to increase linearly 'with time according 
to:	 VbT =	 V1 
where V b e r i is the base-emitter voltage of 1 1 ,
 
1 the 
current produced by T 3, and t time. When V b e Ti 
equals the "on potential" (i.e. forward bias potential) 
(continued overleaf) 
This document was prepared under the sponsorship of the National
	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States
	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19690000491 2020-03-16T18:06:22+00:00Z
of T 1 ,
 
T 1 turns on and T 2 turns off due to cross 
coupling. Thus, the duration of the Off time of T 1 is 
L given by ti = (V be + V i)—where V be5 the for-
ward bias voltage drop of T 1. Similarly, the duration of 
the off time of T 2 is given by t 2 = (V be + V2)1. 
The total duration of a cycle of the oscillator is then 
T t1 +t2 =	 (V1 + V b ) +(V2 +Vbe). 
If C = C2, we have I =	 (V i + V 2 + 2V b e). 
Since V, and V2 are derived from the differential 
amplifier T 5, T6, T 7, T8, R,5, R 6, R 7, R8, R9, 
the sum V 1 + V 2 is constant. Thus, The fequency of 
operation is constant, but the duty cycle, or equiva-
lently, the off time of T 1, is a linear function of V 1 
(see previous equation for t 1). V 1 is in turn a linear 
function of the control input signal. The duty cycle is 
thus controllable at constant frequency. The diodes D 1 
and D 2 serve to decouple the charging of capacitors 
C i and C2 from the power supply +V. Resistor R 16 
and Zener diode D 3 provide a reference input so that 
the duty cycle is a function of the difference between 
the control input and the reference voltage of D .
The circuit may, therefore, also be used as a pulse 
duration modulator. 
Notes: 
1. The operation of the circuit is unchanged if a more 
complex differential amplifier is used instead of T , 
16, T 7 , and T 8. Resistor R 9 may be replaced by a 
transistor current source network similar to transis-
tor T, R 3 1, R 3 2, and R Any other constant 
voltage source may be used in place of D 3 or it may 
be replaced by a resistor voltage divider network. 
2. Inquiries conêerning this innovation may be di-
rected to:
Technology Utilization Officer 
Goddard.Space Flight Center 
Greenbelt, Maryland 20771 
Reference: B69-10512 
Patent status: 
No patent action is contemplated by NASA. 
Source: John Elson Johnson of 
University of Michigan 
under contract to 
Goddard Space Flight Center 
(XGS- 10033) 
Brief 69-10512	 Category 01


Constant-frequency, variable-duty-cycle multivibrator

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