Hello everyone,

I'm a mechanical engineer that, out of necessity, learned some electronics for a project and eventually liked it.
So, in other words, i'm a beginner in all of this.
So I came with a project to execute an audio amplifier.
As I read that an amplifier had a pre-amplifier stage and a power amplifier stage, I come up with the idea to make:
1 - A pre-amplifier stage to boost voltage with a TL071cp op-amp.
2 - A power amp emitter-follower stage to boost current with a MPSA13 NPN transistor.

In that manner I could reach a good power for the output.

So, after I assembled my circuit, some problems appeared.
I attached the circuit and the outputs of the oscilloscope below.

The circuit is shown in Figure 1. The input is a 100 Vp, 500 Hz sinuidal wave, below on Figure 2. The gain on the op-amp stage is set to 50 and the output is a 5 Vp as shown in Figure 3, as expected. But when I entered in the emitter-follower stage the output seems to be distorted. The output voltage can't be seen in the oscilloscope (don't know why) and the current is seen in Figure 4, with a ratio of 1 V/mA.
So I'm asking to the community to help find the problems with the circuit in question. Also some bibliography about audio amplifiers, ideas and improvements if possible would be appreciated.

 

Good attempt, but 100V is going to kill your amp!
Yeah, I know. Typo. Should be 100mV.

Your scope output shows 200V. Where is that coming from? That cannot be a typo.

Have you considered using an LM386 for your output stage?
Or do you insist on rolling your own with discrete transistors?

 

As a matter of fact, I am going to buy some LM386, but I find it a cheap way to do it.
So I'll insist a little with transistors.

 

Ok. Fair enough.
Go figure why the simulator is showing 200V on channel D.

Next thing. You have a 68Ω output feeding a 4Ω load. Not good!

Take a look at some push-pull output designs.

 

That's pretty good.
Why this pot connects directly to the output. And why the need of a PNP transistor?

 

Note that this is a simple example of many similar designs.

Two transistors are needed for push-pull operation. A one-transistor design requires the transistor to be always conducting wasting power. That is called a class-A amplifier.

With push-pull operation (class AB or class B) only one transistor needs to be conducting and this makes it more efficient.

The 1kΩ resistor and 20kΩ pot in series create negative feedback. This has many advantages. It improves linearity (less distortion) and prevents thermal run away, i.e. prevents your transistors from burning out.

 

I appreciate for your replies, MrChips. I can see one can control the voltage output in this arrangement. Is there a way to control the current output somehow in the push-pull?

 

The best way to control the current through the output transistors is to put a small resistor (about 1Ω to 3Ω) in series with the emitter of the transistor.
This has the added advantage that it also introduces negative feedback which is good for balancing and stabilizing the output.

 

Ok, so I think it is 1 resistor between the emitter of the NPN and 1 resistor between the emitter of the PNP transistor to keep the symmetry, am I right? How I know the max max current this assemble can produce?

 

That is the worst audio amplifier circuit I have ever seen!
The lousy old (50 years old design) 741 opamp has a max output voltage swing of about 8V peak-to-peak with a 12V supply when driving the little output transistors. The output of each little transistor will swing to 3.2V peak and try to produce a current of 400mA peak into an 8 ohm speaker. But their maximum allowed current is only 200mA, the 400mA would destroy them. The amplifier produces terrible crossover distortion because the output transistors are not biased properly.

The maximum output power from an LM386 is about 0.5W like a cheap clock radio.

 

As I stated before, I am going through the process of learning. So information would be much appreciated, lectures informing, books, etc. The transistors I choose could handle 500 mA, that I checked. I could switch the op-amps, but what should I check? Higher slew rate, higher output voltage? How would I properly bias the transistor?

 

Here is an amplifier circuit you can try.

 

Here is an amplifier circuit you can try.

Why doesn't it have global AC and DC negative feedback like all other audio amplifiers have?

 

Why doesn't it have global AC and DC negative feedback like all other audio amplifiers have?

This is a simple audio amplifier for experimenting. It is posted for beginners to appreciate push-pull output, DC biasing and negative feedback with emitter resistor. There are no claims made here on the validity, fidelity, and suitability of this circuit.

You can add the negative feedback as desired.

 

That is the worst audio amplifier circuit I have ever seen!
(some text removed for clarity)

Then you should have seen my fist few attempts

Given that he is just starting out @Bruno Furtado is to be commended -as opposed to many first attempts at original audio amplifier designs, his would probably work!

 

MrChips' circuit in post #4 could be significantly improved by using a low-noise rail-rail op amp that, such as an OPA197, and adding bias diodes and resistor, such as shown in post #12 to minimize crossover distortion.
The op amp output would connect to the junction of the two diodes.

 

The circuit in post #4 was copied off the internet. It was not my design.
(I suspected that AG would complain about the LM741 ).

The circuit in post #12 I threw together in 10 minutes. I would have liked to use a pair of complementary darlingtons such as TIP120/TIP125 but I didn't have any on hand.

I just thought that a single 2N3904 is a lot simpler to wire up as a driver than an op-amp.
Using an op-amp instead would be the next step while experimenting with this along these lines.

 

@DickCappes, thanks for the praise, I'm trying my best to understand all the design available for power amps, with the help of @MrChips.
@MrChips, as a matter of fact I bought the exactly same transistors on that design, I should try to assemble it and the others as (firstly) educational purpose. I don't know why, I'm not managing to simulate all these circuits on the softwares. I should try soon enough.
Thank you all in advance.

 

You can add the negative feedback as desired.

I think he (Audioguru) is adding negative feedback

That's pretty good.
Why this pot connects directly to the output.

Don't trust me because I'm just a noob, but the alternative would be to not connect the output to anything and connect the wiper to A. In this case if the wiper failed to make mechanical contact with the carbon ring of the pot there would be an open circuit. By connecting the output and the wiper together, even when the wiper doesn't make contact, there still will not be an open circuit. I think this is the reason.

So information would be much appreciated, lectures informing, books, etc.

Have a look at this great site:
http://sound.whsites.net/

 

I think he (Audioguru) is adding negative feedback

Nah, Audioguru is a positive guy. He's lost a few electrons.