NPN Common Base Amp

[max1000000]

I am trying to understand the single npn transistor common base amp. I have seen several variants of the basic curcuit but rarely with actual resistor and cap values for me to use in an experiment. The ones I have found don't work very well. I'd like to have an input of around 10 mv and I'd like to be able to vary the resistors to obtain different gains.

Can anyone help me get a start here?

 

[kchriste]

I dug up this one from my old tech school experiments book. The impedance at the emitter of the transistor is around 26hm: and the voltage gain from emitter to collector was calculated to be 286 and measured at 257. This amplifier is only good for amplifying a signal from a low impedance source such as a speaker used as a microphone etc.... The 1K resistor on the input is just used so the amplifier isn't over driven if you source it with a signal generator for testing. You would eliminate the 1K if the input source was a speaker-as-mic as noted above.

 

[Roff]

I dug up this one from my old tech school experiments book. The impedance at the emitter of the transistor is around 26hm: and the voltage gain from emitter to collector was calculated to be 286 and measured at 257. This amplifier is only good for amplifying a signal from a low impedance source such as a speaker used as a microphone etc.... The 1K resistor on the input is just used so the amplifier isn't over driven if you source it with a signal generator for testing. You would eliminate the 1K if the input source was a speaker-as-mic as noted above.

See single transistor audio amp for a discussion (on 2nd page) of the derivation of the dynamic emitter resistance, and how its value is a function of emitter current.
Your maximum gain will be approximately Rload/(Rsource+Re), where Rload is the collector bias resistance in parallel with your (probably ac-coupled) load, and Rsource is the impedance of your speaker, in the case mentioned by kchriste. You can reduce the gain by adding resistance in series with the source.

 

[max1000000]

Thanks guys

That got me going. With some minor variations(R1=0, R2=~36K-50K, R3=~28K-35K) I get a gain of order 250X for 1mv/1MHz AC signal.

I see that the gain washes out and the phase shifts as I approach 10MHz.