Basic Audio Amplifier

Posted on Feb 6, 2014

You can clearly see the three stages found in many audio amps (and op amps today). However, looking at actual audio designs, you`ll find the basic topology has been tweaked and enhanced in a few key locations. Here`s a quick tour of design improvements of the audio amplifier. There`s an enhancement for each of the three stages: Differential, Voltage Gain, Output Buffer. Why bother messing with

Basic Audio Amplifier
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the design Higher gain and better output drive lead to a lower distortion performance. Click on the links below to find out more. The Current Mirror (Q8, Q9) doubles the gain of your differential amplifier. It also balances the current in each transistor (Q1, Q2) nicely. Both high gain and balancing helps lower the overall distortion. Replacing the collector resistor of the gain stage (Q3) with an Active Load (Q10) raises the voltage gain and provides better current drive to the positive half of the output buffer. Driving a low impedance like 8 ohms demands a lot of current from the output buffer. A Darlington Output Buffer helps deliver the current and minimize the load on the gain stage. This keeps the overall gain high and the distortion low. – Advanced Audio Amplifier CIRCUIT INSIGHT Let`s check the bandwidth. Does it meet the 600kHz calculated above Run an AC Analysis and plot the output V(20). What happened to the bandwidth It seems to have fallen dismally short of 600 kHz! Turns out, the problem lies in the output stage. Because it`s biased class B, the output transistors are essentially OFF when the output is near 0V. What`s the remedy I`ve snuck in a voltage source VOFF at the base of Q2. A small voltage here will offset the output by the same voltage. Hopefully, a few tenths of a volt will turn ON the output stage. Temporarily set VOFF to 0. 5V and rerun the AC analysis. How high is the bandwidth now Set VOFF back...

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