Design of a VGA for commercial TV preamplifier

[jmpac]

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.

 

[chuckey]

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

 

[jmpac]

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

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).

 

[chuckey]

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