Saturday, June 23, 2018

high-low corner and frequency response

The half power point is .707 signal out or when Xc = Rl in a RC coupled circuit. I will show some RC circuits and some transistor amps to examine.
50 Ohm response rolls off below 90MHz
Looking at the low corner.
Here I have 10 mv in and 5 mv out

My 2N3904 roll off is less than 50 MHz

Changing the caps shifted it slightly above 50 Mhz
usiong 50 Ohm I/O

600 in 50 out

50 in 600 out

600 in 50 out and smaller cap

50 in 50 out and small cap. observe the scaling.

You can see the RC circuit response is predictable but adding a transistor will cause the curve to shift. The transistor parameters come into play and must be accounted for. Some things are beyond the designers control. So we need to use the manufactures data sheet to see if out transistor will serve our purpose.

It's a simple circuit you can type into SPICE is a minute or two but playing with it can reveal a lot about how the coupling system responds.

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