Good day everyone,

I am designing an audio amplifier with a power output stage to provide 1W to a speaker with a load resistance of RL = 8 ohm. I want to amplify a 250mV signal to 4V using an amplifier and pass that 4V through the power amplifier to achieve the 1W power output. I have successfully managed to get both circuits working separately, but for some reason when I am cascading the amplifier with the power output stage, the output is totally wrong and I get 14,32W output power. I am not sure what is causing this and any help or advice on this will be appreciated. I attached both circuits and their models.

 

Sounds like too much gain in the circuit.

Post the asc file of the amps connected together.

 

Sounds like too much gain in the circuit.

Post the asc file of the amps connected together.

Hi, the file Power_amp has both together.

 

How are you measuring the output power?
You seem to have 4V peak, which is 2W peak, 1W average, although it does look rather distorted.

 

Your circuit can't work properly since there is no DC path for the base current of Q3.

 

Use negative feedback to reduce the gain and improve linearity.

 

How are you measuring the output power?
You seem to have 4V peak, which is 2W peak, 1W average, although it does look rather distorted.

The output power is measured over the RL load of the power amplifier. In LtSpice you can use ctrl+left click to display power and then alt+left click on the cursor of the graph to display power average and rmspower

Your circuit can't work properly since there is no DC path for the base current of Q3.

I am not sure I understand? I have the amplifier cascade to the power output stage which is connected to the base of Q3?

Use negative feedback to reduce the gain and improve linearity.

This could work, but I am not sure if gain is the issue. I would first like to figure out what is causing the problem before I tackle it. Thanks anyway.

The output power is measured over the RL load of the power amplifier. In LtSpice you can use ctrl+left click to display power and then alt+left click on the cursor of the graph to display power average and rmspower

If I measure the power of the output stage al on it own, I do get a 4V output and a average power of 1W which I desire. But when I cascade it with the amplifier that amplifies it from 250mV to 4V I get very bad results when I measure over RL.

 

I am not sure I understand? I have the amplifier cascade to the power output stage which is connected to the base of Q3?

But the coupling is capaitor coupled.
You have a current from Q5 which flows through R1, R2 and Q1 to the base of Q3, but then it has nowhere to go.

 

But the coupling is capaitor coupled.
You have a current from Q5 which flows through R1, R2 and Q1 to the base of Q3, but then it has nowhere to go.

Yes I see, but I do not know why it is happening. This structure is common for audio amplifiers so either it is a circuit design flaw or something else. That is why I am here, to figure out what might be the source of the problem.

UPDATE: I looked into a few thing and I might have found the main problem. I believe it might be due to Input loading factor. My output impedance is very high for the amplifier stage and this might cause a horrible ILF. I designed a BJT emitter follower circuit to help increase the input loading factor. It did not make the circuit better though, Files are attached.

 

Looking back at post #1, you seem to have the output load connected at the wrong side of the output capacitor.
Also 10uF would only be a good choice of output capacitor if you wanted treble only. 1000uF would be a better choice for full range audio

 

Actually, I thought the same at first look, but the circuit is okay, The supply is bipolar and the resistor returns to the center ground. No caoacitor is needed.

Bob

 

I am not sure I understand? I have the amplifier cascade to the power output stage which is connected to the base of Q3?

Yes I see, but I do not know why it is happening. This structure is common for audio amplifiers so either it is a circuit design flaw

There is DC bias current flowing out of Q3's base, but it has nowhere to go, so Q3 is biased off.
Yes it's a significant design flaw.
The general structure may be common, but your detailed structure is in error.

I designed a BJT emitter follower circuit to help increase the input loading factor. It did not make the circuit better though,

Not surprising since there still is no DC path for Q3's base.
You've been told what the problem is and you ignore it.
If you don't understand why Q3 needs a DC path, then you need to go back and study the basics of electricity.

 

There is DC bias current flowing out of Q3's base, but it has nowhere to go, so Q3 is biased off.
Yes it's a significant design flaw.
The general structure may be common, but your detailed structure is in error.
Not surprising since there still is no DC path for Q3's base.
You've been told what the problem is and you ignore it.
If you don't understand why Q3 needs a DC path, then you need to go back and study the basics of electricity.

I do not appreciate you insulting my knowledge of electronics. I am not going to discuss this topic further with you. Thank you.

 

Your circuit can't work properly since there is no DC path for the base current of Q3.

Hey - I know that.

ak

 

I do not appreciate you insulting my knowledge of electronics.

He didn't. He evaluated it correctly.

Mind your manners, sparky. You have 13 messages, while Wally has 30,000. That is information. I suggest you put it to good use.

Think #1: Your knowledge of electronics *is* incomplete, and not adequate to the task. Otherwise, you would not be on a public forum asking complete strangers for help.

Think #2: Two different people have given you the same advice because that advice is correct. If you don't understand it, explain *in detail* what you don't understand.

ak

Note: There are several electronic circuit help forums, and this is the nicest one. Any time you want to get slapped around for real, post that kind of sass on Stack Exchange.

 

Actually, I thought the same at first look, but the circuit is okay, The supply is bipolar and the resistor returns to the center ground. No caoacitor is needed.

Bob

Yes, in theory, but there seems to be a large offset on the output.
So, either it needs proper DC feedback (which would include the DC path mentioned), or an output coupling capacitor

 

I am sorry about my reply earlier crutschow and analogkid. I get easily offended when I am not 100% sure of my own answer and frustrated. I have added feedback by changing the current mirror into resistors which still provide the correct current bias. I believe now current from Q3's base does have a DC path now. I attached the circuit.

 

You can eliminate Co2. It is in series with other capacitors, and waaaay too small in value. For example, it forms a high-pass filter with R9. And that corner frequency is . . . ?

The buffer stage has a transistor pulling up (a relatively low impedance), but nothing but a 10 K resistor pulling down (a relatively high impedance). This is driving the very low impedances of R1, R2, R3, and R9. For negative half-cycles of the audio signal, R9 and R10 form a voltage divider of approx. 99%.

ak

 

Yes, in theory, but there seems to be a large offset on the output.
So, either it needs proper DC feedback (which would include the DC path mentioned), or an output coupling capacitor

Agreed. I should have said it needed no capacitor if properly biased.

Bob

 

Agreed. I should have said it needed no capacitor if properly biased.

And keeping the output bias voltage near ground to avoid significant DC current through the speaker would require DC feedback from the output to the bias circuit, which the TS's circuit does not have, so a series capacitor at the output is needed.