Mini amp circuit

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Bob Scott
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Re: Mini amp circuit

Post by Bob Scott » Tue Sep 29, 2009 10:26 am

CeaSaR wrote:Since we are headed this way, I have some LM324's that I was going to use for a giutar amp.
No. While I was editing my post to warn against multi-amp packages, you were posting. The output transistors get their base drive from the power supply connections of the drive op-amp. With multi-amp packages, it would draw too much idling current AND the signals from the other aamps would add and interfere.
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CeaSaR
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Re: Mini amp circuit

Post by CeaSaR » Tue Sep 29, 2009 11:44 am

OK then, how about a couple of '741's? I think The Shack has them in stock.
I think they even have TIP41's and '42's also.

In looking at your designs, it looks like the final output pair is driven by the
power supply connections of the '071. I'm not sure of this as I don't have the
spec sheet in front of me. Am I seeing this correctly?

Studiously,
CeaSaR

EDIT: Duh! You say that the output transistors are driven by the PS connections
of the op-amp! How does that work? Is it the leakage through the transistors that
allows the op-amp to function when hooked up that way? I'm semi-confused on
that, but highly curious...
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Re: Mini amp circuit

Post by Bob Scott » Wed Sep 30, 2009 2:34 am

CeaSaR wrote:OK then, how about a couple of '741's? I think The Shack has them in stock.
I think they even have TIP41's and '42's also.
You can also use TIP31 and TIP32.
Is it the leakage through the transistors that allows the op-amp to function when hooked up that way? I'm semi-confused on that, but highly curious...
Good op-amps are internally biased by constant current sources, so the only current they draw is about a constant milliamp or two, plus whatever current goes in and out of the output terminal. Constant current sources have infinite impedance, so op-amps are also highly immune to power supply noise, so some signal on the supply leads is not going to feed back interference.

If I shorted the output of an op-amp to ground and then attempted to drive its output positive, it would draw a great deal of current from the positive supply terminal in an attempt to raise the output voltage. But instead of shorting the output, we can load down the output terminal with a resistor and use the current that the op-amp is drawing from its + supply terminal to supply current to that resistor, use that same current to turn on the base of the top TIP42 output transistor.

The bottom of the circuit works the same way to drive the TIP41. Then there are feedback resistors to keep the output signal linear with a gain of 100.
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Re: Mini amp circuit

Post by Bob Scott » Wed Sep 30, 2009 2:50 am

I am continuing on a second message because the message editor goes screwy if you try to post a message more than ~25 lines long.

You have to use an op amp that uses all constant current sources. Also, the quiescent current draw of the op-amp from the power supply should be low, like a mA or so. This idling 1 mA is drawn from the base of the TIP42 output transistor, which amplifies it by about 50. So, you always have about 50mA of current flowing through the output transistors. Slightly less would be better, because this current warms up the transistors. If the power across each power transistor is more than a watt, use heat sinks. [email protected] is not too bad, 300mW idling, each transistor. P = I * V.

I might improve this for more power with less idling current but it would add more parts. The beauty of it right now is that it is so simple.

I used this simple design a long time ago in 1980. It made a nice dependable monitor amp for an edit bay.
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Re: Mini amp circuit

Post by CeaSaR » Wed Sep 30, 2009 9:02 am

OK, I have both the TL071 and LM741 sheets pulled up. For the idle current, I am looking at
the Input Bias Current. TL074 is 65 - 200 pA and the LM741 is 80 - 500 nA. Is this the correct
spec to look at for "Idle Current"?

As to whether the 741 is a "good op-amp", as you say, I know it is one of the most common of
all op-amps (at least in regards to low operating frequency bandwidth - ie. audio). From looking
at the schematics on the spec sheets, I believe it uses a constant current source, but I am not
quite sure. Would you be able to tell me which part of each constitutes the "constant current"
section?

TL071 page 5
LM741 page 4

Like I mentioned elsewhere, I still have A LOT to learn.

Thanks for everything,

CeaSaR
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Re: Mini amp circuit

Post by Bob Scott » Wed Sep 30, 2009 10:09 am

CeaSaR wrote:OK, I have both the TL071 and LM741 sheets pulled up. For the idle current, I am looking at the Input Bias Current. TL074 is 65 - 200 pA and the LM741 is 80 - 500 nA. Is this the correct spec to look at for "Idle Current"?
No. Bias current is the current that flows into each input while the amp is operating.
Icc is the "idle current", the power supply current that flows through the amp's power supply terminals while it is idling. It is also the current that will be flowing through the power transistor base connections in the amp schematic. From the spec sheet you supplied, that is 1.4 mA for the TL071. The TIP41/42 power transistors amplify this current by the transistor current gain, the "Hfe" in the transistor's data sheet also known as "Beta". Beta or Hfe is simply a number like 50. This means that 50 times more current will flow from collector to emitter than you make flow from base to emitter. This ratio (50:1) is the current amplification of the transistor. So if you pull 1.4mA from the base, 50 times that current will flow out of the collector.
As to whether the 741 is a "good op-amp", as you say, I know it is one of the most common of all op-amps (at least in regards to low operating frequency bandwidth - ie. audio). From looking at the schematics on the spec sheets, I believe it uses a constant current source, but I am not quite sure. Would you be able to tell me which part of each constitutes the "constant current" section?
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Re: Mini amp circuit

Post by Bob Scott » Wed Sep 30, 2009 10:44 am

Image

I have marked some parts with red numbers for reference in this diagram.

1. This is a depletion mode field effect transistor. It normally requires reverse bias in order to turn it off. When it has its Gate shorted to its Source connection, it becomes a constant current source, so it can supply steady current to the zener diode marked "2" in red. Being driven by a constant current, the zener voltage will be very steady indeed. Note that the constant current from the FET is not adjustable, and might not be exactly the amount of current that the designer needs for this IC.

The constant voltage from the zener is supplied to the base of transistor marked "4", which draws current through resistor marked "3" at a very steady voltage. Since resistors are voltage-to-current converters (and vice versa) the current through this resistor is also constant. By choosing a value for this resistor, the designer now has exactly the constant current value that he needs.

This current flows through transistor marked "5", and this transistor's base automatically adjusts to exactly the right voltage (with respect to its emitter voltage) necessary to draw that specific constant current through that particular type of transistor.

Transistor marked "6" is identical to transistor "5", and it's base is at the same voltage as transistor #5, so the current flowing from emitter to collector of #6 is identical to that of #5. This whole circuit is called a "current mirror" because the same contant current flows through #6 as #5, and can be replicated many times just by adding more identical transistors as needed and connecting their bases to the same spot.

Transistor #7 does the same thing, or should do, but the designer has added a resistor to the emitter. This would produce a current that is less than the other stages I mentioned. This is new to me. Learn something new every day!
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Re: Mini amp circuit

Post by CeaSaR » Thu Oct 01, 2009 5:33 am

Bob,

Looking at the spec sheets again - under ICC this time - I see that the 741 is rated between
1.7 and 2.8 mA. If The Shack doesn't have TL071's (or similar), which I highly doubt they will,
I'll pick up a 741 instead. And whichever of the transistors they have.

Now as to my last schematic on the 3rd page, you said that it doesn't have good negative
feedback. How would you rearrange/change the existing setup to correct that?

Thanks,
CeaSaR
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Re: Mini amp circuit

Post by Bob Scott » Thu Oct 01, 2009 10:26 am

CeaSaR wrote:Now as to my last schematic on the 3rd page, you said that it doesn't have good negative feedback. How would you rearrange/change the existing setup to correct that?

Thanks,
CeaSaR
I'll look but it will take a bit of tweaking to the design. Right now it looks like R3 and R4 are set up as negative feedback to give the amp 14.4 X gain. But, that's like putting a cruise control in a model T Ford and setting it for 200mph. It ain't going to get there. There just isn't enough gain there to limit it to 14.4 with feedback.

I will get back.
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Re: Mini amp circuit

Post by CeaSaR » Thu Oct 01, 2009 11:04 am

Bob Scott wrote:...cruise control in a model T Ford and setting it for 200mph.
What??!! Are you telling me that I didn't get the chopped, channelled and lowered 429 SCJ model T?
Is mine the south field find with a 3.5 HP Briggs and Stratton? :shock: :P :mrgreen:

I actually don't have that high of an expectation of power output from it. I just would like to see
your take on what is "not quite right" and how to remedy those items.

Thanks,
CeaSaR
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Dean Huster
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Re: Mini amp circuit

Post by Dean Huster » Thu Oct 01, 2009 4:41 pm

I don't understand why C5 is in that circuit at all. Bypassing the collector resistor on a common emitter circuit doesn't make sense except if you do want deliberate high-end roll-off. I'd take it out altogether and see what the circuit does.

Edit: If this response makes no sense or if it's a repeat of someone else's comment, I apologize. I just realized that I had originally intended to hit another thread topic and missed, hit here instead my old brain saw page one and thought it was on the only page or something ... geez, I hate senior moments. Actually, they're longer than moments these days ....
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Re: Mini amp circuit

Post by CeaSaR » Thu Oct 01, 2009 5:03 pm

Hi Dean,

I found the basis for this on the web here. From that schematic I played with it and, with the help of those
who have contributed so far, have come up with the current version. From everything discussed already, it
was decided that it was originally put there for top end roll-off, presumably because the amp was originally
designed for voice or equivalent from a ham shack.

I have the circuit loaded in CircuitMaker right now and am flipping back and forth between C5 in and out. Not
much difference in the output. It gains approx. 2 mW at 1kHz and the waveform is pretty much the same. It
probably could be left out and not do much harm. I think I checked it before and saw not much difference then,
but I don't remember why I left it in. I think when I breadboard it, C5 will be left out as that's a value I don't
remember in my stash.

Thanks,
CeaSaR
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Dean Huster
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Re: Mini amp circuit

Post by Dean Huster » Fri Oct 02, 2009 4:04 pm

But the effective value of the collector load (4.7K) will be halved at only 338 Hz! That'll be a significant change in gain and at a really low frequency. I take it that this wasn't designed as a traditional "audio" amplifier?
Dean Huster, Electronics Curmudgeon
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R.I.P.

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Re: Mini amp circuit

Post by Bob Scott » Fri Oct 02, 2009 6:10 pm

Dean Huster wrote:But the effective value of the collector load (4.7K) will be halved at only 338 Hz! That'll be a significant change in gain and at a really low frequency. I take it that this wasn't designed as a traditional "audio" amplifier?
f = 1 / (2 * Pi * 4,700 * .01E-6)

I get a low pass filter with 3,386 Hz as the -3dB corner frequency, and that's just about right for amateur radio voice communication. He is a ham. I looked at the peekchers on his site. He does mention the filtration of highs is optional and that you can delete the cap(s). One decimal point over, Dean!
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Re: Mini amp circuit

Post by CeaSaR » Fri Oct 02, 2009 9:04 pm

To all,

I came across another (hopefully) helpful website. It's called The Amplifier Institute. Would any of you please
take a look and see if it is another good place to learn about amplifiers? Of course I think you guys have really
helped me a lot aready and I think you (collectively) still have a lot that you could teach me.

Thanks again,

CeaSaR
Hey, what do I know?

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