Basic NPN/PNP Amplifier test circuit not behaving as expected in multisim can anyone please tell me

Basic NPN/PNP Amplifier test circuit not behaving as expected in multisim can anyone please tell me what I have done wrone?The function generator sweeps nicely between +-20mv but the bc547 and bc557 do not behave as expected, I expect I have done somthing stupid can anyone point out to me what it is?

So I think I fixed the PNP side but can someone with experience please check, it won't travel into the positive on the output of the transistors.

Matthew...ok, for starters your PNP transistor (the bottom one) is the wrong way round, the emitter (with the arrow) should be connected to the emitter of the top transistor, and the collector to -6V.  This will form what is called a complimentary push-pull amplifier.  BUT (and it's a big but) you have no bias on these transistors so they will not start conducting until the input voltage exceeds 0.6V from ground (the midpoint of the batteries).  It will work if your input signal is above 0.6V peak (1.2V peak-to-peak) but you will have a very distorted output with what is called crossover distortion - where the signal crosses from positive to negative you will get a "dead Zone" where neither transistor conducts and so you'll get a nice horizontal kink in your sine wave at the crossover point.This is very instructive - and will show you why hi-fi amplifier designers take such care with the biasing of their amplifiers.  I have multisim so if you still ahve problems email me the multisim file: stn564 (at) yahoo (dot) com (dot)  au.Your next step once you get it working will be to bias the transistors properly to get rid of the crossover distortion.cheers // David

Matthew - I am at work now but can fire this up on multisim when I get home.As I said your input should be a fair bit above 1.2V Pk-Pk for you to get any results.Your graphic is very small (this is a problem on EEWeb which we are trying to have sorted out.Also if you can email me your mutisim file I can have a play with it as well, will save me a fair bit of time entering it againWhat I would expect to see is something like the following (assuming you are using a sine wave input):

Mathew I got a similar circuit going in Multisim at home and it works fine - too well - it shows no crossover distortion at all !  Which is wrong!  I don't have time to breadboard it but I would like to - if Multisim is incorrect I would be surprised, but I tried a few things and could not get it to distort.   send me your email and I'll send my Multisim files.  Cheers // David

Dave, the simulated circuit has no load on the output. Try adding a 10K-ohm resistor from the emitters to ground.

Thanks Pete but I did try a 1K to ground from the output and also a 1K from the bases to ground, no joy.  It's as if the generator is giving a current drive instead of a voltage drive.  I might try it with a DC source instead of the generator and see what happens...  but I am torn between curiosity and a serious lack of spare time...

Perhaps this says more about spice than anything else. No one has said what the voltage drive is from the input generator, aside from at the start, where 20mV was mentioned initially. My guess is that if you have a large drive voltage, you should see that the p-p output voltage swing is somewhat less than the input, related to the Vbe, but the cross over gets lost in the step size and interpolation of the plotting routine (or something like that)If you put a low drive voltage on the input, say a volt or two p-p, then the distortion will become apparent.Cheers,Richard

Richard...I beg to differ!   I reduced the drive to almost nothing, even tried adding diodes in series with the transistor bases...always a perfect sine wave at the output.  As you say, think it is more SPICE than the design.  I can see why the late great Bob Pease didn't care much for SPICE.....

I checked using LTSpice out of curiosity,  it shows no distortion with a large drive voltage, but the output voltage is lower by the Vbe, if I drive it with a volt or two, the distortion is very apparent. No bias, no resistors, just the default NPN and PNP transistors.Cheers,Richard

So is the 'correction' i posted correct? Or did I get that wrong and had it right the first time? I will post the file in a few mins.   NpNPnP

I'll assume the 2nd one I posted is correct then.