opamp input power amplifier - problems

[sunrise]

guys i have problems to make this amplifier work...what am i making wrong - it seems to me that i have problems with the input part of the amplifier....
can somebody help me....

[sunrise]

help needed....

[lumanauw]

Is it oscilating? Try 2x47pf from top and bottom of VBEmultiplier (Q12) to opamp's output (pin 6). The cap values may be experimented in the real cct.

[sunrise]

i made it in sim using Multisim 8....i have arround 13kV at pin 6 (and going up) of the opamp... because of the input problems (i assume) the complete amp is not is normal working point but i can't figure what is the problem....
when reading in simulator the bias current through output transistors is 19A....

right those values will never be reached in real life - my amp will be burned before they reach it to be measured....
but i can't figure what is the problem.....
is the input configuration o.k. (arround opamp)

i do not know - i am not sure....

please advise what to do
thanks
sunny

[sunrise]

here are some values on some points....

[andy_c]

Originally posted by sunrise
i made it in sim using Multisim 8

One thing to check is your op-amp model. Many simulators including LTSpice use the ancient Boyle model for op-amps. The Boyle model will not simulate correctly with this type of design, because it has the very unrealistic property that the supply current does not change when the load current changes!

Try simulating the op-amp in a circuit by itself and see if the supply current changes with the load current. If it doesn't, you'll need another op-amp model.

[sunrise]

i am new in simulating things....

in oder words You want me to make new file where i will put an opamp in, lets say, inverting mode and to monitor if opamp supply current is changing with load current ?!

[sunrise]

i have made this circuit.....
for square signal at the input that is 1V i have 10V at the output....and the currents of the power supply seem to be steady and the current from the output seem to vary from 46.3mA upt to 50mA....

what to do ...
is my Multisim stupid or am i stupid... is everything working fine or we are not sure here.....
in other words this circuit might work without a problem but we will not know until i make it in real life and not in sim...

killing me softly with this sim


please advise thank you in advance

[andy_c]

Originally posted by sunrise

i am new in simulating things....

in oder words You want me to make new file where i will put an opamp in, lets say, inverting mode and to monitor if opamp supply current is changing with load current ?!

Sure. Put a load of, say, 500 Ohms on the output and supply a DC input voltage so that the DC output is, say, positive 5 Volts. Then the load will be drawing 10 mA. This 10 mA should be coming from the positive supply, in addition to its normal supply current.

In the Boyle model, the supply draws a fixed current as the load current changes. This makes it so that power amp simulations like yours don't work, because it's the varying supply current that's driving the external devices.

[andy_c]

Originally posted by sunrise
i have made this circuit.....
for square signal at the input that is 1V i have 10V at the output....and the currents of the power supply seem to be steady and the current from the output seem to vary from 46.3mA upt to 50mA....

what to do ...
is my Multisim stupid or am i stupid...

This is a bug in the Boyle op-amp model that your simulator is using. Try to find out how to use vendor-supplied op-amp models in your simulator. The models from Analog Devices are usually good (but not Linear Technology - they use the Boyle model also).