Page 206 - Complete Wireless Design

Amplifier Design

Amplifier Design 205

Figure 3.109 A simple but low-cost VGA circuit.

as the gain is decreased. Design the amplifier and the attenuator circuits as
described in the various attenuator and amplifier sections.

A low-cost variable-bias VGA amplifier (reverse gain control, Fig. 3.110)
1. Choose R to drop half the V when the transistor is at its desired gain:
C CC
V

CC

2
R
C I
C
2. Choose an R of 10 kilohms.
B
3. Find the voltage required at the AGC (AVC) port of the amplifier that
causes the base current (I ) to create a collector current for the desired
B
full gain. The transistor’s characteristic curves in its data sheet will con-
tain the information on I versus I , while the current gain graphs will
B C
show the I versus h :
C FE
AVC (I 10 kilohms) 0.7
B
4. Choose the limits of AVC voltage that will supply the required range of gain
by substituting the desired I in the above equation with the minimum and
B
maximum I -related gain values.
B

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