Gain question on a simple circuit.

Discussion in 'General Electronics Chat' started by daviswe, Jun 17, 2009.

  1. daviswe

    Thread Starter Member

    May 14, 2009
    13
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    I have a situation perhaps someone here can help with. I'm currently testing a circuit (see attached screenshot) I designed from some hints and some knowledge. A simple two transistor mic preamp, and the trick is that its powered from the microphone input line from the system. The mic input line has 4.5VDC on it, and I've successfully made a preamp that takes my 5-15mV dynamic mic signal and boosts it as high as 160mV, which is plenty for the application, but could be higher on another system I use if I can figure the best way to increase the gain with this low voltage I have to work with. I need to make it an adjustable gain to be able to apply this same circuit to other systems as well. VOX thresholds are as high as 100mV on some systems, for example, and as low as 14mV on others.

    So, looking at the resistors in the circuit, I'm thinking R8 and R3 set the gain of Q1, and perhaps R1 sets the gain of Q2? I admit to some guessing here. The R7 is there to limit current, and the amp is very sensitive to changes in it or in R9, which I use to represent the internal input impedance of the system the mic plugs into.

    To control overall gain, I'm thinking make R7 a variable resistor. To increase the max gain of the TPv2 node, I'm thinking I need to tweak both transistors to perhaps operate at sqrt(40) gain each so I get a max gain of 40 (voltage gain, that is). Hence my question about which resistors really control the gain of the two stages. Admittedly, this has been confusing since it's not a typical topology for me. For example, is the 220K R2 feedback resistor really needed??

    I'm open to suggestions on a better design, and would even pay for a solid one if anyone can offer it.

    Many thanks,

    Ed
     
    Last edited: Jun 17, 2009
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Interesting design, better presentation than most. I'll be following this thread with interest.
     
  3. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Try changing Q1 emitter bypass capacitor C5.

    Lower capacitance, lower AC gain & less distortion.
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Did you notice the even-harmonics distortion at the output of the preamp? The top of the sine-wave is "skinny" and the bottom is "fat".
    A transistor (and a vacuum tube) does that when the gain is high and the negative feedback is low.
     
  5. daviswe

    Thread Starter Member

    May 14, 2009
    13
    0
    Yes, I did notice that distortion, in fact it can get a lot worse, and I think it has to do with the signal source. I'm using a laptop computer headphone jack to drive a single 75-Ohm speaker in a tube with a port 3" away where the microphones are placed for testing. If I crank the laptop volume all the way, I get this distortion very pronounced, but if I keep it midrange, it's good. I ordered a signal generator from Digikey already... :) Good point about the neg feedback and high gain. I'd like to get this biased up so each transistor stage is working at sqrt(gain), perhaps.

    Also, it's possible that the dynamic mic may be seeing some DC bias across C1 which would cause a kind of clipping from the coil being physically biased to one side from the DC. I've actually turned a mic off before by biasing it with DC to the point it no longer moves with SPL applied.

    I'll test with a 0.1uF on C5 and see how that changes...and waiting on the sig generator!

    Ed
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    896
    Look at this distortion of a single transistor that has no negative feedback. The distortion is so high at high levels that the output level canot be measured. The distortion is higher at higher output levels.

    The distortion with negative feedback added is low.
     
  7. daviswe

    Thread Starter Member

    May 14, 2009
    13
    0
    So your 3% distortion result is from removing C2? I can see where C2 might cause waveform shaping especially if it's a large capacitor...
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    The emitter resistor provides negative feedback when the capacitor across it is removed. Then the gain is reduced from about 200 with the capacitor, to 9.1 without the capacitor. Then the 40% distortion drops to 3% distortion.

    The large value of C2 causes it to be a dead short at signal frequencies so it does not shape the waveform.
     
  9. daviswe

    Thread Starter Member

    May 14, 2009
    13
    0
    I've done some parameter sweeps with the sim (5Spice, which I paid for today, and am waiting on my serial, it's a good app!) and about all the gain I can get is 10x voltage gain, which may be ok, but I need to make the mods shown in the attachment and do the testing again to see how valid it is, and also see if the lower C5 value improves the distortion. It's a voice comm on a motorcycle, so studio quality is not as important as volume and 'vocal contrast' over the background noise.

    I've tested the 'Motorcycle Input' input impedance at various frequencies, and it shows 50K and higher, but I realize now that needs to be done at very low signal voltage levels, and not with the kind of output this amp delivers so that I get a small signal impedance at perhaps 1Khz and not the reactance from higher freq and signal levels, hence the "???" in the blue box. I'm just not sure what's inside the system, but assume there is the obvious 4.5V supply, some kind of resistance (perhaps should be parallel rather than serial as shown), and probably capacitive coupling.

    Next, I swept R1 and found more gain, about 10% by changing to 250K from 100k, but the ckt in sim is not sensitive to other resistor values, except R7, the coupling resistor, and without it, the amp can't make gain since Q2 emitter is at 4.5v, and 68K sims the best gain.

    Thanks again for any input. I probably should move this to one of the project threads??

    Ed
     
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