Class AB amplifier

222

04:58:37

Basic class AB amplifier, question for anybody who knows more than me. Im calculating the current gain at 40. Can anyone verify that and furthermore, tell me if that is a typical gain for this type of amp? The current measurements on the input I took were on an input cap, output was taken across 27ohm resistor. 

published 11 years ago

hurz

11 years ago

Because this is not a pure Class B (not much but a little class A as well) the current gain is lower than just the transistor beta Hfe. The diodes together with the input cap + 2.2kohm are building the input impedance. 0.74V across each diode by 2mA is about 380ohm. With 2.2kohm as bias resistors gives a little lower than transistor hfe beta. This is valid for small signal and will change over input peak voltage. You propably noticed the unsymetric output signal form, heavy distotion. This is because the current across your diodes is quite low and does not set both transistors at good operating points. Just decrease the resistors to 1kohm improves this already a lot and increases the class A behavior a little to all in all a class AB. Hope this helps.

jpollack

11 years ago

It does, decreasing the input 2.2k made a huge difference, thanks.

kyitharoo

10 years ago

thank you

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