Hi, im currently designing a class AB amplifier to drive two 8 ohm speakers, but I am not able to understand the concept of impedance matching and input/output impedance. My block diagram flows from an input signal(cassette player) -> preamp and tone control-> AB amp->speakers. From my understanding, some kind of impedance must match the speakers impedance to avoid signal distortion. Any advice would be greatly appreciated.

 

Impedance matching is required for maximum power transfer. It is also required for HF (high frequency) applications to prevent signal reflections in transmission lines. It has little to do with signal distortion in audio frequency applications.

In general, you want the load impedance to be 10 times higher that that of the source. For example, if the output impedance of the cassette player is 1 kΩ, you want the input impedance of the preamp to be 10 kΩ or higher. In this way, the effect of the load on the source signal is 10% or lower.

In the case of driving a loudspeaker, if the impedance of the loudspeaker is 8 Ω, you want the output impedance of the amplifier to be 8 Ω or lower.

 

I am not able to understand the concept of impedance matching and input/output impedance.

That seems to be a rather common problem.
It can be confusing as to how/where impedance matching is actually needed.
Setting the load impedance to match the source impedance for maximum power transfer is only done for a fixed source impedance.
If you can vary the source impedance, then you set it as low a possible to transfer the maximum power to the load.

The input impedance should be higher than the source impedance as MrChips stated, since we are interested in maximum signal voltage transfer, not power transfer.

Matching the output impedance to the speaker impedance to maximize speaker power, typically with a transformer, is only done for tube (valve) amps, since tubes have a high intrinsic output impedance, whereas transistor emitter-follower output impedance (typical of solid-state audio amps) is low.

Thus the output impedance of a solid-state amp is normally very low, (much less than an ohm) to minimize any distortion or frequency response affects the typical change in speaker impedance with frequency would cause, and provide the desired high damping-factor to the speaker.
The amp's rated speaker impedance determines the optimum amp's design output voltage and current for a desired power output, and is not related to the amp output impedance.

What is the desired output power of your amp design?

 

Impedance matching is required for maximum power transfer. It is also required for HF (high frequency) applications to prevent signal reflections in transmission lines. It has little to do with signal distortion in audio frequency applications.

In general, you want the load impedance to be 10 times higher that that of the source. For example, if the output impedance of the cassette player is 1 kΩ, you want the input impedance of the preamp to be 10 kΩ or higher. In this way, the effect of the load on the source signal is 10% or lower.

In the case of driving a loudspeaker, if the impedance of the loudspeaker is 8 Ω, you want the output impedance of the amplifier to be 8 Ω or lower.

Thank you MrChips, that has improved my understanding of impedance in general.

 

That seems to be a rather common problem.
It can be confusing as to how/where impedance matching is actually needed.
Setting the load impedance to match the source impedance for maximum power transfer is only done for a fixed source impedance.
If you can vary the source impedance, then you set it as low a possible to transfer the maximum power to the load.

The input impedance should be higher than the source impedance as MrChips stated, since we are interested in maximum signal voltage transfer, not power transfer.

Matching the output impedance to the speaker impedance to maximize speaker power, typically with a transformer, is only done for tube (valve) amps, since tubes have a high intrinsic output impedance, whereas transistor emitter-follower output impedance (typical of solid-state audio amps) is low.

Thus the output impedance of a solid-state amp is normally very low, (much less than an ohm) to minimize any distortion or frequency response affects the typical change in speaker impedance with frequency would cause, and provide the desired high damping-factor to the speaker.
The amp's rated speaker impedance determines the optimum amp's design output voltage and current for a desired power output, and is not related to the amp output impedance.

What is the desired output power of your amp design?

that makes sense thank you.
Well about desired output power, im aiming for 0.3W with a 4.5V power supply. 4.5V isnt enough because I have measured 0.09W output power across the speaker.

 

For a classic AB audio amp, the power output is directly related to the power supply voltage.
If your power supply is 4.5 V, the best amplitude signal will be 2.25 V. When you take into account the collector-emitter voltage drop, you are more likely to see about 1.6 V amplitude.

The RMS (root mean square) voltage is 1.6 X 0.707 = 1.1 V
Power deliver into 8 Ω load is 1.1 V x 1.1 V / 8 Ω = 0.15 W

Please post the circuit schematics of your amplifier.

 

my version of the amp involved the 3 transistor configuration. The values i calculated for the components (resistors and C1) here are more trial and error based then logical calculations, I am not sure if my thinking process was right.

 

hi iL,
Please post your LTSpice asc file.
E

 

If you want 0.3W into 8Ω you will either need to use a bridge type amplifier, which will increase the power by about a factor of 4, or increase your supply voltage to at least 6V.
What's your preference?

How concerned are you about distortion as that circuit's will be significant.

 

With that low a voltage supply the power output IS rather limited. Adding an output transformer can provide a bit of more power,especailly if you change to a push-pull output stage. BUT that is a rather big-deal change.

 

You are on the right track. Increase your supply voltage to 9 or 12 V. Your resistor values are two low. The circuit will draw too much current.




Here is the same configuration with an extra preamp stage.

 

hi iLEE,
You can use this LTSpice method for finding Zin and Zout of your amplifier.
E

 

hi iL,
Please post your LTSpice asc file.
E

 

If you want 0.3W into 8Ω you will either need to use a bridge type amplifier, which will increase the power by about a factor of 4, or increase your supply voltage to at least 6V.
What's your preference?

How concerned are you about distortion as that circuit's will be significant.

supply voltage is capped at 4.5V as its part of the requirement. Therefore, I will try to implement a bridge type amplifier.

 

You are on the right track. Increase your supply voltage to 9 or 12 V. Your resistor values are two low. The circuit will draw too much current.

View attachment 372258


Here is the same configuration with an extra preamp stage.

View attachment 372259

My supply voltage is strictly limited to 4.5V, so im trying to work with that by possibly adding a bridge type amp. About resistor values, how did you calculate yours? as calculating component values is my weak point. Could you please give me some advice in how to get appropriate component values?

 

hi @iLEEZi
As you can see I created an asc file from your circuit, I used a different TIP model, but the Zin/Zout are the same. Post #13.

If you need an explanation of the LTS impedance method, just ask.
E

BTW: is this project a School or College assignment.?

 

hi iLEE,
You can use this LTSpice method for finding Zin and Zout of your amplifier.
E
View attachment 372272

Thank you very much, its very helpful.

 

For your amusement, below is the LTspice sim of a relatively simple six transistor circuit, with the really good feature of using a differential input stage to provide both output DC feedback to stabilize the output bias at near 1/2 the supply voltage, and output AC feedback to set the circuit gain and reduce distortion.
It's the simplest circuit I would consider for building an audio amp with decent stability and performance.

The simulation shows <0.5% distortion at 0.8W output with a 9V supply.

View attachment 372266

Thank you. This circuit seems complex based on my current understanding of audio amplifiers. If you dont mind me asking, how did you go about calculating the values for the resistors. I understand that I need to set a Q point for the transistors, but im not entirely sure on how to calculate that.

 

hi iLEEZ.

Is this project a School or College assignment.?

Moderation

 

hi @iLEEZi
As you can see I created an asc file from your circuit, I used a different TIP model, but the Zin/Zout are the same. Post #13.

If you need an explanation of the LTS impedance method, just ask.
E

BTW: is this project a School or College assignment.?

Yes, I would love to have an explanation for the method. And also yes it is an university group project, im assigned to build the AB power amplifier stage. My initial idea was just understanding Zin/Zout and Z matching, but people were kind enough to also assist me with the amp circuit.