# Class AB amplifier BJT operating point question

Lets say we have a diode biased class ab amplifer. As known at the output, npn handles the positive part and pnp handles negative part of a signal. This bjts are operating between +/-12V and output voltage is 18Vpp. VCEs of the output bjts are are around 12V. Is it possible to get same output(18Vpp) if we change operating voltage to 0-12Volts?

I am asking because of confusion. From the Vce there is a swing of 18volts. At +/-12 supply voltage thats ok but since each transistor amplifies half portion of the signal the swing for each part will be 9 volts and at 0-12V operation if I set Vces to 6Volts the swing will 4.5V upper 4.5V lower for each signal?

Best.

"Is it possible to get same output (18Vpp) if we change operating voltage to 0-12Volts?"
- No, because you can't get a voltage swing larger than the supply voltage in such an amplifier; it is usually slightly lower than the supply voltage.
Using 12V, you would probably get about 10.8Vpp at best, due to the base-emitter voltage drop on the output transistors (Vbe).

• Thank you very much and also what happens if AC and DC load lines does not intersect what does it mean?
– oggy
Commented Apr 16, 2020 at 10:07
• I haven't heard about that one before, and I don't know enough at the moment to answer that question. Commented Apr 16, 2020 at 10:33

Is it possible to get same output(18Vpp) if we change operating voltage to 0-12Volts?

No, the amplifier cannot output a voltage outside of its power supply range minus a few volts of margin determined by output transistors headroom voltage required for proper operation.

A typical diode bias class AB emitter follower will output up to about 1.5V from the supplies, so its output voltage range would be 1.5-10.5V with single +12V supply. That's enough for 9V peak to peak with 6V DC offset. With a single positive supply the output can't go to 0V.

However you could use two amplifiers, one of them inverting, and wire them as a bridged amp.

In this case you will get 18V peak to peak. This is very common in car audio where you only get a +12V power supply.

• What if I would like to make only one channel like mono guitar amps? Or Can I use a transformer to compansate the output voltage difference? 18V@+/-12 and 9V @0/12V
– oggy
Commented Apr 16, 2020 at 13:11
• what is your goal? an audio amp? Commented Apr 16, 2020 at 18:50
• yes but for guitar
– oggy
Commented Apr 16, 2020 at 19:43