Electrical – Choosing transistor for amplifier

amplifiertransistors

I'm going to design amplifier, requirement is that impedance for in should be more than 20kohm, impedance for out should be less than 50ohm, and voltage gain should be more than 200. So I'll make cc-ce-cc amplifier, but how can I choose transistor for each cc, ce, cc? And is it better to chooose transistor first and after that choose Rc, Re…and then R1, R2…etc? Or is there any other better way(order) to design it? Any guide will be appreciated. Thanks

Best Answer

Voltage gain for a single bipolar device, with no emitter resistor, is at most VDD/0.026. You want 200, thus you need at least 5 volts. Go for 12 volts.

For a Rin of 20,000 ohms, assuming Beta of 100, you need reac of 200, which means Ic of about 100 microAmperes.

To swing the collector voltage by 10 volts at 100uA, you need 100K ohm Rcollector.

To interface that to 50 ohms, you need a lot of emitter followers (CC topology).

Given Rcollector is 100Kohm, and assuming 20pf (two of Cob) on that node, your timeconstant is 2 microSeconds, 500,000 radians per second, or about 90,000Hertz F3dB.

Any small-signal bipolar will do fine: 2N3904, 2N918. Even Nchannel JFETs, if the transconductance is high enough.

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