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Circuit lets AGC amp hold its gain

-January 19, 2015

AGC (automatic gain control) amplifiers are frequently used devices. Their gain is a function of the level of their input signal. The main function of AGC amplifiers is compression of a signal, but sometimes the gain needs to be temporarily frozen. This Design Idea provides just this function.

Let's examine this idea with Analog Devices’ SSM2166 as an example. The SSM2166 is a microphone preamplifier with variable compression and noise gating whose transfer characteristics are in Figure 1 and typical application diagram in Figure 2.

Figure 1  Compression and gating characteristics with 10dB of fixed gain


Figure 2  SSM2166 functional block diagram & application circuit

The chip has one interesting feature which isn’t presented in the data sheet: we can control the gain using an external voltage applied to pin 8 (AVG CAP). Table 1 shows the observed level detector voltage (pin 8) vs. input voltage. A simple circuit which provides gain-hold capability to the SSM2166 by fixing the output voltage of the internal level detector is shown on Figure 3.

Table 1  Level detector voltage (pin 8) vs. input voltage (observed dependence)

Input

dB

Level detector

VDC

0

4,00

-10

3,56

-20

2,88

-30

2,28

-40

1,80

-50

1,41

-60

1,00

-70

0,60


 

Figure 3  Circuit provides hunting and holding gains for SSM2166

Auto-tracking of the voltage level on the AVG CAP is done by sample-and-hold amplifier IC1 (LF398). This level is duplicated on Hold CAP if IC1.8 is high – the SSM2166 is working in compression mode. When you want to hold the gain, drive this pin low. IC1 will then hold the voltage level on Hold CAP, and analog switch IC2 (ADG417) will connect IC1’s output to pin 8 of the SSM2166, overriding the level detector.

This Design Idea works with other types of AGC amplifiers, too [1]. But in this case, an analog switch is unnecessary. In these circuits, a sample-and-hold amplifier should be connected between the level detector and voltage controlled attenuator.

 

References:

  1. Rentyuk, Vladimir “Use of an AGC Amplifier as a Soft Limiter of Signals”, Electronics World, May 2010

Also see:

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