# emitter follower biased by input signal?

For the standard emitter follower you typically need input coupling capacitors and a DC bias of roughly half the supply voltage however what if the input signal itself swings around half the supply voltage?

e.g if the previous stage to an emitter follower is a standard op amp non-inverting amp with a split supply (+12V,-12V) with the input referenced to centre point but the succeeding emitter follower is referenced to -12V (i.e +24V, 0V), so The op amp output is referenced to ''+12v'' relative to the emitter follower? Could this replace the input coupling cap and resistor biasing or would it not work?

example of circuit is attached below

Differential emitter follower?

Inverting amp?

• Draw the full idea of the circuit to stop ambiguity and sentences that make no sense. Apr 26 '20 at 11:03
• I did.what sentences make no sense?
– Jay
Apr 26 '20 at 11:08
• but the preceding emitter follower - what preceding emitter follower? Crop your diagram to fit the page to make it legible, Apr 26 '20 at 11:12
• thanks for correction, it should be succeeding. also fixed size of image... ms paint is not the best editing software
– Jay
Apr 26 '20 at 11:15
• The board has a built-in schematic editor, which is better to use than paint for schematics. Apr 26 '20 at 11:20

Could this replace the input coupling cap and resistor biasing or would it not work?

A more convenient idea would be this: -

It's called a DC coupled amplifier and uses feedback directly from the emitter to set the gain at the output to be: -

$$1 + \dfrac{R2}{R1}$$

Of course, if you really want the return current for your headphones to go via -12 volts (potentially noisier) then you will need an output capacitor.

• The issue is I cant really modify the op amp configuration as its part of the output buffer on a DAC PCB. I was thinking there may be another option making use of the differential or ''balanced'' output of this DAC. A bridged amplifier that would recieve this differential signal from the DAC does not reference its output to ground, the headphone/speaker is basically 'floating' between the outputs of each half the bridged amp. I added updated version of schematic to show what I mean.
– Jay
Apr 26 '20 at 13:51
• This will not work because the NPN is in the loop and adds open loop gain making the circuit unstable. The NPN will have to be a PNP in that case and resistors will have to be added/adjusted. Apr 26 '20 at 15:23
• @le_top I don't think so - it's an emitter follower and has a voltage gain less than unity and is quite likely to have a frequency response many times greater than the op-amp hence, instability won't happen. If it were a MOSFET source follower, you'd have a more realistic argument. Apr 26 '20 at 15:33
• Ok, my look at the circuit was a bit too fast. Sorry. Apr 26 '20 at 16:42
• @Andyaka I added a version of your circuit to the OP that using inverting amp, do you see any issues with that?
– Jay
Apr 30 '20 at 0:25

If you mean ...

simulate this circuit – Schematic created using CircuitLab

then with suitable op-amp biassing so that its output swings somewhere between the rails, the emitter follower will 'follow' the output at about 0.7 V lower.

• yes, thats it just if the op amp is configured as a non-inverting amp/buffer. So the question is would referencing the op amp input to ground take care of biasing?
– Jay
Apr 26 '20 at 11:45