Clipping Problem

Discussion in 'The Projects Forum' started by PRS, Apr 28, 2009.

  1. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    This amplifier clips at 24.5 volts peak to peak. When I turn up the gain, both the high-going part and low-going part of a sinusoid clip at the same time.

    The supply is +/- 19 volts. Vc=0.5 volts. Ve=-13.4 volts. Thus Vce is 13.9 volts and allows for a swing of almost 28 volts peak to peak. Between the collector and the supply is 17.5 volts so this should be no problem for a 28 volt peak to peak swing.

    Why is it clipping, how do I get more swing out of it? Any help will be greatly apprecated.
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    Because of the 180R resistor and because the transistor drops a significant voltage across it.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Vc should be about +2.3V. Why is it 0.5V?

    Also how did you come up with gain=4.4? It should be almost 8.

    I can show you how to calculate the clipping levels, but first I need to understand why the bias and gain values don't make sense.
     
    Last edited: Apr 28, 2009
  4. Audioguru

    New Member

    Dec 20, 2007
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    The 2.2k collector resistor and the very low 4.7k value of the load causes the positive swing to be only to +11.9V.

    I simulated it:
     
  5. CDRIVE

    Senior Member

    Jul 1, 2008
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    What 180Ω resistor? My eyes are old but not that old. I can't read anything on that sch. :)
     
  6. Ron H

    AAC Fanatic!

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    When I click on it, it gets bigger. Voila! 180 ohms!
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
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    OK, I clicked on the thumb but I neglected to click on the print when the thumb opened. I can see it now. Duh! :eek:
     
  8. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    Ron was right about the gain. I rechecked it this morning. It's 8.5 v/v. And I, too, am confused by the voltage at the collector. Looking at the voltages and the resistances at the emitter, it seems there is 7.6mA flowing through the transistor. But when use that value to calculate the voltage at the collector its 1.35 volts! This would mean the base is sucking up alot of current, would it not?

    At Audioguru's suggestion I pulled the load resistor. This resulted in higher gain, but did not give me greater voltage swing there. Ron, I calculate under the assumption that voltage swing is either 2*Vce or Vcc-Vc -- whichever is smaller -- the first having to do with the lower part of the sine, the latter the upper part.

    Audioguru, your simulation seems to have reproduced part of the problem -- the clipping at the top, but I kid you not when I say the bottom clips at the same time as the top.

    Thanks for all the help. I'm going to measure every resistor and use several transistors and tweek this circuit until I get to the bottom of it. I'm curious.
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Here is how I calculate clip levels. I show the simulated results, but the math is simple enough to do. One key is to treat the coupling cap as a battery (2.05V in this case).
     
  10. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    The way I see it, you got the high-going clip point by (Vcc-Vc)*(RL/RL+Rc). I never considered that there was voltage division going on between RL and Rc. That explains it and thanks! I just learned an invaluable lesson!

    As for the negative going part of the sine wave, I see your result being the same as mine, 2*Vce. Is this correct?

    The funny thing is, with the resistor values given I get, experimentally, Vc=0.5 volts! Magic? Incidentally, Ve=-13.4 volts and Vb=-12.8 volts.

    I know this is offtopic, but you sure have a great Spice program! You and also Audioguru and SgtWookie! Gotta get one. I've heard there are demos available for download, but my computer is way too slow to download any substantial program.

    Thanks for your substantial reply, Ron, and thanks to Audioguru, too, for having run it through a Spice program. I'm quite impressed. Gotta get me one. ;)
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    In my sim, Vce (quiescent) is 15.34V. Negative clipping occurs at -13.7V.
    Below is the result of the sim with Vin=3.6V p-p.
     
  12. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    As I said, this seems to be a magic circuit. It doesn't obey the rules of theory. Vce is, experimentally 13.9 volts and this should, I think, allow for a swing of 27.8 volts, except for the limitting condition of the positive swing, which you explained very well.

    I'm beginning to think that this amplifier configuration is only good for (2/3)*Vsupply, where Vsupply is 38 volts (as it will be when the power amp is included). Can you design an amp with this configuration having, say 75% Vsupply? How about 80%. If you can I'd sure like to breadboard it and see it for myself.

    I say this because I have designed 3 seperate circuits of this type. But they all -- every one of them -- get clipped at nearly 25 volts peak to peak.

    Thoughts?
     
  13. mik3

    Senior Member

    Feb 4, 2008
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    Note that this is not a normal CE amplifier, it is called a swamped CE amplifier due to the 180R resistor which makes the output response more linear.
     
  14. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    Right. I had to include the 180 ohm resistor to avoid nonlinear distortion. I'm now driving this circuit with a circuit having a gain of 11. Now I'm seeing ripple voltage at the output. I'll try decoupling with a resistors and capacitors.
     
  15. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The circuit has very poor power supply rejection. At 120Hz, the positive supply rejection is only 3.35dB. In other words, if you have 1V of ripple, 680mV will show up on the output. The negative supply rejection is 15dB. For 1V of ripple, 173mV will show up on the output.
     
  16. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    Do you mean the configuration I've chosen is inherently poor at power supply rejection or my particular design is defective? Where do you get the numbers, Ron?

    I've got a total gain of 100v/v from a two-stage of voltage amplifier. Both are CEs with a partially bypassed emitter resistors -- 180 ohms in both stages. The ripple on the supply lines is 40mV peak to peak, but this is without my power amp connected. The ripple, a sawtooth wave, gets amplified such that a 1 volt wave rides the output. This is unacceptable. I have in mind decoupling each amplifier with a capacitor and a resistor, but I haven't got to it yet. If needed I'll use zeners or regulators to bias the voltage amp.

    I got hung up on my power amp -- Darlington class AB. It was working just fine but suddenly quit working. My cat may have messed it up. I get the peaks, but not the valleys. I've replaced components, especially the NPN, and its bias circuitry -- a resistor and 2 diodes,but to no avail. Tomorrow I'll try my last TIP120. I've got to admit, I'm getting a little frustrated. Round and round we go! ;)
     
  17. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Yes, the configuration is poor, without very good supply filtering. If you use RC filtering, or regulators, it will eat into your p-p out swing capability.
    I got the numbers from a simulation.
    The positive supply ripple is attenuated by 4.7k/(2.2k+4.7k), because the collector is a very high impedance (hundreds of kohms). The negative supply rejection may be different than my simulation value, depending on the input coupling capacitance, the emitter bypass capacitance, and the impedance of your voltage source. This was assumed to be zero in the gain calculations, but probably isn't in a real amplifier, certainly not in a second identical stage. Higher source impedance allows some of the -19V ripple to creep in through the lower base bias resistor.
     
  18. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    Then what is the solution? I remember a laboratory project back at WSU where we made a DC amplifier using 2N2222s, with a gain of 1000 and a bandwidth of 2MHz. That high gain was inviting distortion, but we used regulated supplies. Our prof. pretty much gave us an overall design which made use of three differential amplifiers, each decoupled from the supply, and a class AB amp at the output. So I'm thinking the differential amp is probably the best bet in the case of high audio voltage amplification.

    I tested the 100 v/v amp with a microphone and a guitar and, in order to acheive the full 30v p-p capability of my amp needs a further gain of 5. So, should I use Diff amps in the first two stages of voltage gain? (There will be a total of 3 stages for voltage gain since I want a high frequency roll off well above the audible range for the sake of experiments with ultra sonic stuff.)
     
  19. Ron H

    AAC Fanatic!

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    Is this an exercise in discrete transistor amplifier design? If not, toss it and use op amps for voltage gain.:)
     
  20. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    Yes, it's an exercise in discreet transistor design. That's how I learn the nuts and bolts of electronics. ICs are too easy. I'm curious, I sure don't know it all, and I'll be learning until the day I die or the day I get senile, whichever comes first. A project, to me, is all about learning. Once done with it, I go on to another that challenges me. This way I am ever learning never knowing and its all about the process, not the product. It's how I pass the time. ;)
     
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