Browse Prior Art Database

Variable Gain Circuit With Common Mode Control

IP.com Disclosure Number: IPCOM000039191D
Original Publication Date: 1987-May-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 2 page(s) / 40K

Publishing Venue

IBM

Related People

Siljenberg, DW: AUTHOR

Abstract

In bipolar variable gain amplifiers (VGAs), the common mode bias voltage, at the output, is a function of the gain control voltage. Subsequent amplifier stages cannot always tolerate this common mode range. This circuit controls the common mode voltage as the gain changes. The figure shows a circuit of a VGA stage. In this circuit, as +VGAIN increases (-VGAIN decreases), Q1 and Q4 carry an increasing portion of the current from Q5 and Q6, respectively, while Q2 and Q3 carry a decreasing portion. This produces an increasing drop across R1 and R2. Differential amplifier Q8 and Q9 is changing in response to VGAIN. As +VGAIN increases (-VGAIN decreases), Q9 turns on and Q8 turns off. This raises the collector voltage of Q8, and also the emitter voltage of Q7.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 100% of the total text.

Page 1 of 2

Variable Gain Circuit With Common Mode Control

In bipolar variable gain amplifiers (VGAs), the common mode bias voltage, at the output, is a function of the gain control voltage. Subsequent amplifier stages cannot always tolerate this common mode range. This circuit controls the common mode voltage as the gain changes. The figure shows a circuit of a VGA stage. In this circuit, as +VGAIN increases (-VGAIN decreases), Q1 and Q4 carry an increasing portion of the current from Q5 and Q6, respectively, while Q2 and Q3 carry a decreasing portion. This produces an increasing drop across R1 and R2. Differential amplifier Q8 and Q9 is changing in response to VGAIN. As +VGAIN increases (-VGAIN decreases), Q9 turns on and Q8 turns off. This raises the collector voltage of Q8, and also the emitter voltage of Q7. This rise is equal to the increased voltage drop across R1 and R2, maintaining a constant common mode voltage at VOUT. The current source (J2) can be reduced by a factor N as long as the resistors R3 and R4 are N times larger than R1 and R2.

1

Page 2 of 2

2

[This page contains 2 pictures or other non-text objects]