• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

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 !

1.png 2.png 3.png
 

Attachments

Last edited:

rjenkinsgb

Well-Known Member
Most Helpful 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
Most Helpful 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.
 

audioguru

Well-Known Member
Most Helpful Member
Like I said on the other website, all audio amplifiers have negative feedback from their output so that the voltage loss and distortion produced by your darlington transistors is eliminated.
 

Latest threads

EE World Online Articles

Loading
Top