Please Help With Op amp

Discussion in 'The Projects Forum' started by ngong123, Jan 12, 2012.

  1. ngong123

    Thread Starter New Member

    Jan 12, 2012
    What could be wrong with this design?

    Design specifications

    1. Two inputs one plus , one negative
    2. Capable of driving a 50 K ohm load to + 10 V
    3. Frequency response DC - 10MHz + 2 db.
    4. The input impedance must be at least 1 Megohm
    5 The output impedance must be less than 50 ohms
    inches on any side. 6. Temperature range: 25 -- 100oC
    7 The gain must be between 1000 - 1500
    Thanks for your Help

  2. Georacer


    Nov 25, 2009
    What are the symptoms? What makes you think the problem doesn't work? Help us by pointing us towards the direction of the problem.
  3. jimkeith

    Active Member

    Oct 26, 2011
    This appears to be an op amp constructed of discrete components--It probably will not run in a simulator as shown--I believe that it is workable with a few fixes:

    1. PNP transistor Q15 must be inverted so the emitter goes to R4

    2. R4 appears to be excessively high, or R3 excessively low--will prevent the collector of Q15 from proper positive voltage excursions--suggest increasing R3 to 10K

    I am a little at a loss in estimating output impedance--if it is too high, adjust R3 & R4 down while keeping the ratio about the same

    3. Quiescent power dissipation on Q2, Q13 & Q14 is excessively high (230mW)--while it will work OK at 25°C, it will not work at 100°C--suggest increasing current mirror source resistor R11 to 2.2K--check 2N3904 thermal derating to see if this keeps it within max Tj limits

    If you actually build this amp, make sure that Q13 & Q14 are thermally bonded together to make current mirror work over the temperature range.

    4. Current mirror source resistor R10 is off by an order of magnitude--should be 3.6K

    5. R12 must increase from 600K to at least 1M to satisfy input impedance spec--do this also to R13

    6. Differential amp load resistors R2 & R3 are way low--will prevent sufficient voltage excursion to enable Q6 to turn on sufficiently enough to turn on Q15--suggest 1K

    7. The magnitude of R2 & R3 may be subsequently adjusted to bring open loop gain within specifications--my gut feel is that it will be close to spec--other than that, R4 may be adjusted, but it may not increase above 2K without affecting maximum output swing spec

    This should now run in your simulator--you can tweak it from there--good luck!