# Trying to understand class AB amplifiers

Discussion in 'Homework Help' started by frankmash69, Aug 9, 2014.

1. ### frankmash69 Thread Starter New Member

Aug 9, 2014
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0
Hi everyone. I am trying to understand how class AB amplifiers work, such as the one in this picture.

http://imgur.com/TS8WNCE

I understand this type of amplifier is biased so that there is no crossover distortion by having both the transistors slightly on.
My question is how does Q1 ever get its input to the base if there is a diode in the way? Thanks. Any help would be appreciated.

2. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,500
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Notice that input signal is oscillating? In the negative cycle of the input signal the voltage drop across the diode is large, large enough to turn diode ON, this means that there is a current going through the diode. Where does this current come from? It comes from +V. So current leaves +V and arrives to the junction of Base and diode, some current goes into Base, the rest goes through diode.

3. ### MrChips Moderator

Oct 2, 2009
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The diode is not in the way. You are thinking of a diode functioning as a one-way switch that conducts only in one direction, i.e. when forward biased.

Both diodes are forward biased and therefore conducting.

The diodes are functioning as voltage references maintaining a voltage of about 0.6V across the diode. This voltage is what sets up the transistor bias and what keeps the transistors slightly turned on for the AB amplifier.

Feb 17, 2009
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5. ### crutschow Expert

Mar 14, 2008
13,475
3,361
To perhaps help clarify, the diodes are not operating to block reverse current, they are only operating in the forward direction to provide a fixed voltage bias for the transistor bases to generate the desired transistor bias current. Whether they can block current in the reverse direction or not is immaterial to the operation of the circuit.

6. ### studiot AAC Fanatic!

Nov 9, 2007
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Also bear in mind that the transistors operate in push-pull.

That means that in general one is on and the other is off.

In pure class B each transistor operates for exactly half a full cycle of the wave.

However at very low levels a transistor is not properly on so class AB biases the 'off' transistor so this is on enough to assist the 'on' transistor in its weak area.

Doers this rough and ready explanation help?

7. ### crutschow Expert

Mar 14, 2008
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"Assisting" the on transistor is to allow a smooth transition between the turn-off of one transistor and the turn-off of the other (crossover). This prevents crossover distortion (notch or dead spot in the output waveform at the crossover point).

8. ### frankmash69 Thread Starter New Member

Aug 9, 2014
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0
Thanks for all of the replies. They answered my question.