Two Stage Amplifer

[Nigel Goodwin]

Who's saying "DC bias doubles AC RMS output power"? I'm not.

Maybe you missed my point. Why shunt the speaker impedance with a 50 resistor? All I'm saying is remove the resistor and drive the speaker direct. Is this difficult to grasp? I'm sorry I can't explain it better.

Why are you talking about speakers?, there's no mention of driving a speaker in the question at all.

 

[marcbarker]

Why are you talking about speakers?, there's no mention of driving a speaker in the question at all.

The question IMO is moot, based on (I think) a misunderstood point I made to do with driving the speaker.

We'd been talking about driving speakers for the last 9 or so postings.

I specifically asked in #36: "isn't the 50 ohm resistor the actual speaker being directly driven?" and this question was not answered. Therefore, logically there is instead a AC-coupled 50 ohm speaker placed across this resistor, the way I saw it anyway, maybe I'm getting too far ahead.

I said: "You double the output power by having this DC going through the speaker instead of a resistor." Which appears to had been misunderstood. Serves me right for being ambiguous.

I'll try and use more words in the hope it can be understood. Presently, there is DC current going through the 50 ohm resistor. Not shown on the schematic is the actual speaker, via a blocking capacitor (which was not denied when I asked about it). Therefore I am saying that in a practical circuit, the 50 ohm resistor be removed completely, and the speaker be substituted for this resistor. Or in other words, remove the 50 ohm resistor, and make this the speaker instead, with DC bias through it. Or another way of putting it, substitute the 50 resistor with a 50 speaker, directly driving it, instead of a resistor as well.

Because if it is not, a 50 resistor shunts the 50 speaker. I think this one is obvious, so I won't elaborate any more

 

[Nigel Goodwin]

The question is moot, because it's based on a misunderstood point to do with driving the speaker.

We have been talking about driving speakers for the last 9 or so postings.

But is of no help to the OP - but for what it's worth, I would agree with directly connecting the speaker. However, the speaker in the collector would be a far better place - but doesn't address the original assignment (which doesn't even require a speaker).

 

[marcbarker]

But is of no help to the OP - but for what it's worth, I would agree with directly connecting the speaker. However, the speaker in the collector would be a far better place - but doesn't address the original assignment (which doesn't even require a speaker).

I agree. Seems to have gone off on a tangent about speakers and mW.

I wonder if OP is still here? There was some questions asked to him.

 

[audioguru]

The first sentence of this thread talks about the amplifier that drives a 50 ohms speaker.

 

[Nigel Goodwin]

The first sentence of this thread talks about the amplifier that drives a 50 ohms speaker.

And as you are well aware, there's no such thing

If you read the attached DOC file, which is his assignment, it gives no such requirement.

 

[andrebc]

Sorry, for any confusion my assignment caused, the 50 ohms is a load, not a speaker. when I was teating my circuit I was using a 50 ohms resistor! and it was coupled with a capacitor so that no DC flows through it.

So DC bias does not flow to my circuit that's why I was using 3 coupling capacitors and one bypass capacitor. I read in a one of the replies that I don't need a resistor connecting the Collector of the emmiter follower to its base,a nd I just can connect the collector directly to VCC. that's wrong! the BJT will be burnt instantly.

I guess enough about this assignment. I have a new one it is a bout testing a diffrential amplifier, so far the design is fine as well as the simulation. I just one ask this Pspice question, I never wrote nestlist of circuits, all I do I just draw the circuit in the schematics of psice and then run simulation, does any body knows how to write a nestlist. In the schematics attached I am using LM 324 as my opamp but actually I will be using LM 741 instead. Can anybody tell me how to write a nestlist on Pspice? thank you all in advance.

 

[Nigel Goodwin]

Sorry, for any confusion my assignment caused, the 50 ohms is a load, not a speaker. when I was teating my circuit I was using a 50 ohms resistor! and it was coupled with a capacitor so that no DC flows through it.

So DC bias does not flow to my circuit that's why I was using 3 coupling capacitors and one bypass capacitor. I read in a one of the replies that I don't need a resistor connecting the Collector of the emmiter follower to its base,a nd I just can connect the collector directly to VCC. that's wrong! the BJT will be burnt instantly.

It was never suggested that you connect the base to Vcc, it was suggested that you connect the base to the previous stages collector - which is a common design technique, giving many advantages.

 

[marcbarker]

Edit: Oops! Sorry, I duplicated a repsonse..

I read in a one of the replies that I don't need a resistor connecting the Collector of the emmiter follower to its base,a nd I just can connect the collector directly to VCC. that's wrong! the BJT will be burnt instantly.

??? In what posting# did it say connect EF base to VCC?

Even if you did so, it would NOT harm the EF-connected BJT. Granted, connecting base to vcc could maybe stress a voltage-sensitive emitter load, e.g. if it was a low voltage light bulb. But anyway, it didn't say anywhere connect the base to Vcc!

If anything, you still could of connected the 2nd transistor base directly to the 1st transistor, linking out the capacitor and deleting the base resistor. The AC coupling was not really blocking much DC at all, so you might as well had connected the two points together anyway.

I never did understand the reason for AC coupling between stages. OK, if it was an RF amplifier there would be an L-C impedance matching network there. But I understand it wasn't!

 

[marcbarker]

Talking of "direct-connected speakers", take a look at this posting:

https://www.electro-tech-online.com/threads/using-a-more-powerful-speaker.95488/#post767305

 

[Hero999]

**broken link removed**
LM386 Schematic diagrams

 

[marcbarker]

Can anybody tell me how to write a nestlist on Pspice?

yeah, draw the circuit, click "view netlist", then cut and paste the netlist.

 

[andrebc]

yeah, draw the circuit, click "view netlist", then cut and paste the netlist.

That's all? I thought it is much more than that. How can I import the spice model of LM741 to my nestlist?

 

[Hero999]

Why do you want to do that?

The LM741 should never be used for audio - it's nowhere near good enough.

 

[andrebc]

Why do you want to do that?

The LM741 should never be used for audio - it's nowhere near good enough.

LM741 is not for my audio lab, it is for a different project that is a differential amplifier, please see the attachement below. Lm741 will be used as an inverting opamp with unity gain, I will used Vin as and vout as the input to the differential amplifier, and amplify the difference.

 

[marcbarker]

That's all? I thought it is much more than that. How can I import the spice model of LM741 to my nestlist?

I thought the question was "how to write a netlist"?

Isn't your spice already got a 741, or if not, another equivalent you can believe in?

For your diff-amp circuit you can use just about anything can't you? How about a 324?

 

[marcbarker]

The LM741 should never be used for audio - it's nowhere near good enough.

Millions of guitar amplifiers and PA system amplifiers have used 741. Well, if I'm not mistaken, that is. The way you said it's "nowhere good enough", I have doubts now

 

[Hero999]

From what era?

They must be pretty poor quality amplifiers then.

If you look at that the datasheet for the 741, you'll find that the slew rate isn't fast enough for anywhere near full output and the gain is fairly low so there's not much negative feedback even at modest gains.

There again, I suppose guitar and PA amplifiers don't have to be excellant quality.

 

[marcbarker]

They must be pretty poor quality amplifiers then.

That's the whole point of engineering isn't it?, attaining satisfactory performance with available components and down to a price too, and amps stayed that way a long time, right up until it was NO MORE EXPENSIVE to drop in faster opamps. Beatles, Hendrix and Bowie wouldn't of noticed any difference if the amp was wider bandwidth than the speakers could handle. From that era, in terms of record sales numbers per head of population, it has never been repeated.

Anyway, what's the point of extended bandwidth in the music supply chain technology, if the record-buying customer are not aware of anything above 25 kHz? The disc-cutting machine was limited in frequency, so anything higher than that heard would be from the amplifier itself, probably an over-reacting unstable opamap with a feedback loop resonating, that didn't show up in pspice.

 

[audioguru]

The lousy old 741 opamp was used in guitar amps because nothing better was available such a long time ago.
The guitar speakers had no output above only 8kHz. The 741 opamp barely makes it to 9kHz.
The players and listeners were deafened by too loud screeching.