Automatic Gain Control - exercise from 'The Art of Electronics'.

[FullBridgeRectumfire]

Hi all.

Anyone fancy to help me out with this very simple exercise?

The VN13 has an ON resistance (VGS = +5V)
of 15 ohms (max). What is the range of amplifier
gain in the second circuit (assume that the
current sink looks like 100MOhm)? What is the low frequency
3dB point when the FET is biased so
that the amplifier gain is (a) 40dB or (b) 20dB?



How to find the lowest drain-source resistance of the MOSFET? How to find cut-off frequency knowing only a value of the emitter cap or how a change of Rds value affects frequency response?

[danadak]

Assuming the bipolar is high beta, then Ie =~ 1 mA. Vemitter = 6V - Vbe =
~ 5.3V. Re then looks to be 5.7V / 1 mA = ~ 5.7K. You can then compute the ~
3db from 1 uF and 5.7K.

Make the same calcs when the bias changes.


Regards, Dana.

[FullBridgeRectumfire]

Your solution does not include changes of mosfet's characteristic acting as a voltage controlled resistor.