Design of a VGA for commercial TV preamplifier

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Hello all. Does anybody know how to design the typical VGA for a commercial TV preamplifier?
In the picture attached, i dont know if the trimmer (7 to 22 dB) is a variable resistor or a variable capacitor. How does it work?
Thank you.

 
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chuckey said:
You can see its got 270 degrees of rotation, its a pot and it will increase the bias on the RF transistors, so their gain falls.
Frank
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I suppose you meant: if the bias (colector or drain current) decreases, the gain falls.
However it implies lower linearity of the amplifier (output IP3) and that's not like to broadband signals (like several 8 MHz multiplex of DVB-T).
 

no I mean that the bias increases, the collector current rises, the input impedance falls, so reducing the input signal, and the collector voltage falls. using forward bias for junction transistors is a good way of preserving their linearity (much better then reverse bias). its what we used on military pack sets, the current flattening out at 50 mA!!
Frank
 
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