# Explaination required (Audio amplifier Class-AB)

#### ahas1234

##### New Member
Hi everyone !
I'm new to electronics.

I was trying to design AB-push pull audio amplifier (using Darlington). for load current 1.6A (rms) across 4ohm

My input peak is 9v (peak). but at output, my voltages reduces to 8.66V (peak). Wjile required is 9v peak (6.4V rms)
And power despite of around 20Watt (peak), is around 18Watt peak only.

Can someone please provide me a reason, why does this happen ?
Or any way to fix this out !

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#### rjenkinsgb

##### Well-Known Member
To get 1.6A through a 4 Ohm load, you need 4 x 1.6 = 6.4V RMS.

The amp must be capable of providing a peak voltage across the load of at least 1.414 x that, so more than 9.05V positive and negative.
You need a higher input voltage swing.

The transistor base current will reduce and the input moves away from 0V. You could add capacitors from input to base either way (across the diode pairs) or try "bootstrapping", split R1 and R2 into two series resistors each, with capacitors from the amp output to the resistor junctions.

#### ahas1234

##### New Member
To get 1.6A through a 4 Ohm load, you need 4 x 1.6 = 6.4V RMS.

The amp must be capable of providing a peak voltage across the load of at least 1.414 x that, so more than 9.05V positive and negative.
You need a higher input voltage swing.

The transistor base current will reduce and the input moves away from 0V. You could add capacitors from input to base either way (across the diode pairs) or try "bootstrapping", split R1 and R2 into two series resistors each, with capacitors from the amp output to the resistor junctions.
Actually, i'm trying to amplify a 200mV (peak) input to 9V (peak) using (non-inverting opamp first ).

I set gain to 45. so output becomes 9V.
which goes as input to AB amplifier. (to get theoretical output of 9V peak from AB after amplification).

you mean, i should increase the gain ? to provide more input voltage signal to amplifier?

#### rjenkinsgb

##### Well-Known Member
Yes, a bit more gain should give you the 1.6A RMS; the output voltage needs to be at least 9.05 peak.

The other mods are to get nearer 1:1 input to output and probably improve linearity a bit.