Radio frequency amplifier to distribute reference frequencies within communications systems and provide automatic gain control with low phase distortion is described. Use of photocell in electronic circuit to produce desired conditions is explained. Operation of entire amplifier circuit is analyzed

Lutes, G. F., Jr.

English

NASA Technical Reports Server

September 1972	 B72-10245 
•	 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 
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be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Broadband RF-Distribution Amplifier
o+v 
Power Supply 
___ --- Ov 
	
03	
C5_ f 
	
04 1I 	 L'	 Output CI	 02 
RI
C2r -. R3
T2	
D Input 
if	
£fl T
	
R4	
r 
R7 
	
LL__U*r	 i 
Photocell	
!-------
tor
R D2 
The problem: 
To design an RF-amplifier which distributes refer-
ence frequencies to various places in communication 
systems and provides 10 db of automatic gain control 
(AGC) with low phase distortion (+3 to +13 dbm 
input power); the output amplifier stage also must 
supply 180 mffliwatts into 50 ohms at an output im-
pedance of 50 ohms resistive. 
The solution: 
Utilize a photocell in a variable attenuator con-
figuration to obtain low phase shift with AGC; broad-
band transformers are used in a feedback network to
obtain a variable output impedance with low power 
loss over a wide frequency range. 
How it's done: 
The first stage of the amplifier shown in the dia-
gram consists of Q1 and associated components, and 
serves to isolate the photocell attenuator and the 
AGC lamp from the input; it also provides a 50-ohm 
input impedance at the input (J1). The first stage is 
followed by the variable attenuator which is a photo- 
cell in an L network consisting basically of Cl, Ri, 
and B.2. 
The second stage (Q2, Q3) is a simple amplifier 
(continued overleaf) 
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that provides isolation between the attenuator Ri 
and the output stage Q4; this stage reduces the react-
ance at the output of the attenuator Ri and helps 
reduce phase shift as it is varied by the AGC power 
dissipated in the lamp. 
The output stage Q4 is a power amplifier which 
supplies 180 milliwatts. Two types of feedback are 
provided by the transformer configuration at the out-
put of Q4. The feedback from the collector of Q4 to 
its base, through R3 and C2, lowers the output im-
pedance of Q4 while the feedback from the trans-
former T2 developed across R4 and coupled to the 
base raises the output impedance. The potentiometer 
(113) between these two points can be used to provide 
a variety of adjustments between the two kinds of 
feedback. The output impedance can be varied from 
about 10 ohms to about 100 ohms, thus making it pos-
sible for the novel distribution amplifier to accommo-
date a wide range of loading. In the present applica-
tion, the adjustment is made for 50±2 ohms. The 
capacitor C5 is in a lead network which cancels the 
reactive component on the output of the amplifier. 
The RF output level also passes through CS into 
a detector (115, R6, C4, and Dl) which generates a 
DC voltage proportional to the RF level. The DC 
voltage is compared to an adjustable reference volt-
age (D2; 117) and the difference is then amplified and
phased properly to drive the lamp. The intensity of 
light falling on the photocell attenuator (Ri) controls 
the input of the RF output; the desired output level 
is set by R7. 
The amplifier was used to drive a passive hybrid 
power divider which split the power eight ways and 
provided eight outputs at 20 milliwatts (1 volt rms) 
each. Phase noise is so small that it cannot be meas-
ured accurately with instrumentation ordinarily avail-
able in development laboratories. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP 72-10245 
Patent status: 
No patent action is contemplated by NASA. 
Source: George F. Lutes, Jr. of
Caltech/JPL
under contract to

NASA Pasadena Office

(NPO-11401) 
B72-10245	 Category 01


Broadband RF-distribution amplifier

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