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Impact of load resistor on transistor audio amplifier

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Uros11

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Hello everyone.

I am building an audio amplifier using transistors, based on a circuit taken from "Art of Electronics". In the book, the circuit is presented without load resistance, and I understand quite well how the circuit works in general. In my case, I need to have an 8 Ohm resistor at the output, and I can notice that the output voltage is cut. I changed the circuit a little bit and experimentally changed some values, and in that case I can get some better sine wave. I want to know what is the impact of 8 Ohms on the whole circuit, and what are calculations for getting resistor values, because I don't feel good if I know how to get there experimentally, but lacking theory.

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The circuit isn't well suited for 8 ohm load, output current is limited by Q5 gain and R6.
 

Yes, in the meanwhile I changed my Q4 and Q5, since they were small signal BJTs and now the circuit is working properly, but I would like to know how in general load affects this circuit and what can be done to maximize the gain?
 

It's a simple topology with rather limited performance. You find better suited audio amp circuits in literature.
 
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Uros11 said:
Yes, in the meanwhile I changed my Q4 and Q5, since they were small signal BJTs and now the circuit is working properly, but I would like to know how in general load affects this circuit and what can be done to maximize the gain?
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There is the circuit gain, which is voltage_out/voltage_in but I think you mean how to get the maximum undistorted voltage out.
That's basically determined by the current gain of the output transistors and the current that R6 can provide during the negative excursion.
The problem is that as the output goes more negative, R6 can only supply less and less current.
Bootstrapping R6 from the output can significantly increase the drive.
Lookup bootstrap audio output.
 

Bootstrapping advantage is that it doesn't need additional transistors. Example circuit is also in "Art of Electronics".
 

Here is a similar circuit except the input and drive transistors are the opposite polarity.
The bootstrap is provided by C6 and D1 to R12.
Note the bootstrapped voltage (yellow trace), which goes well above the supply voltage so R12 can still provide base current to Q5 at the positive peak of the output.

For your circuit the bootstrap cap would go to R6, with the diode going to the negative voltage (cathode).

Capture.PNG
 

In general, 3 diodes to bias class-AB common-drain stage is not too good idea I think. In post #1 the transistors dissipate more than 9W in idle state. Be cautious. They wil burn.

To maximise the closed-loop gain (A_cl) of the feedback system change the feedback resistor divider's ratio (R4 and R5). A_cl will has a maximum value over your desired bandwidth, it will be limited by the open-loop gain of the circuit (A_ol). To increase A_ol is hard, there are many trade-offs (consumption, linearity, dissipation, cost, physical size) and maybe only with other elements it is possible or an else circuit structure.
 

As others have mentioned, this circuit and output transistors are not suited to drive an 8 ohm load. Regarding open loop gain, you can increase gain by replacing the collector resistors in the input stage and VAS with constant current sources, or with bootstrapping.
 

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