Reviewing Basic amplifier stage properties, I found unanimously that a Current Amplifier has a Very Low (ideally zero) Input Impedance and a Very High (ideally infinite) Output Impedance. The Emitter Follower's Input/Output impedances however show totally contrasting properties (High Input and Low Output impedance), and from small signal models its quite easy to see that the Emitter Follower does provide a significant current gain (β+1). Quite naturally I am not being able to comprehend these conflicting facts, from an intuitive viewpoint. Does it have to do with the fact that it is a voltage buffer? Please help.
Electronic – Emitter Follower as a Current Amplifier
amplifieremitter-follower
Related Solutions
If you really need to mux the inputs, how about something like this?
Since your DAC output is already DC biased, if the bias it uses is provided on another pin, you could eliminate the cap and resistor for the DAC output, and use that pin as the bias for the other input. Otherwise use one of these:
Depending on what your inputs and outputs are connected to, you might need grounded resistors on the other side of the coupling caps, RF bypass caps to ground, etc.
You'll never get am LM324 to do what you want so why not bite the bullet and use an LM386. It's minimum supply is 4 volts and at 5 volts can produce 4 volts peak to peak. It's intended for audio applications as well and can drive a 4 ohm load (or greater): -
Read the data sheet if you want more details and other application circuits.
Best Answer
You need to do a small-signal analysis of the input impedance for signals above the base-emitter diode voltage (~.6 volts). In this regime, you'll see that a small increase in voltage will produce a large current increase, which is the mark of a low impedance.
Likewise, if you hold the base current constant, you'll see that the emitter current does not vary much for variations in load resistance. And this is a mark of high output impedance. Granted, for a single transistor it's not outrageously good, since beta varies with current and collector-emitter voltage.