Hello All,,,

I am trying to build the circuit developed in this discussion (see attached) on a breadboard before puting on a PCB. I have the first stage of amplification working i.e until the first opamp. But as soon as i connect the emitter follwer transistor part at the output of the 2nd opamp to amplify current,,,i get a square wave as the output of the 2nd opamp. I have check all the connections thouroughly and I dont understand whts going wrong...it all seems to work fine on LT spice...

Any suggestions as to wht cud be wrong wud be great as i am really stuck!!!

 

sorry find the circuit attached here

 

If the opamp U3 output is going to the rails, then it's not getting feedback from the output of the emitter follower, or R7 is missing. Perhaps the load of the coil is too much. Try disconnecting L1 from the circuit, if it's connected.

Note that D1 needs to be thermally coupled to Q1, and D2 needs to be thermally coupled to Q2. Epoxy (JB-Weld) would work well.

 

Hi,,,
the output of the opamp is cuming out to be a square wave of pk-pk 4v.. so i assume the output is not clipping and the feedbak to the opamp -ve terminal from the emitter follower is pretty much nothing,,,,but wht could be going wrong??? yes R7 is connected fine as well

 

Since this arrangement is not working,,,i have decided to use a poweramp in place of the second opamp and remove the emitter follower arrangement...so could any1 tell me how to make an LT spice model for L272...Thanks!!

 

Hi,,,
the output of the opamp is cuming out to be a square wave of pk-pk 4v.. so i assume the output is not clipping and the feedbak to the opamp -ve terminal from the emitter follower is pretty much nothing,,,,but wht could be going wrong??? yes R7 is connected fine as well

OK, try connecting the output of the opamp to the inverting input - see if that cleans up the sine wave of the output.

Just use a jumper, or short piece of wire. If you then get the proper waveform, start checking to see if there's a signal at the junction of D1/D2, then a similar signal on the base of Q1 and Q2 (but offset slightly in voltage), and then see if there is an output at the junction of the emitters of Q1 and Q2.

 

Since this arrangement is not working,,,i have decided to use a poweramp in place of the second opamp and remove the emitter follower arrangement...so could any1 tell me how to make an LT spice model for L272...Thanks!!

I've been looking around for one; haven't found one yet.

Neither ST Microelectronics nor ONsemi has a model for it; not on Yahoo's LTSpice group either.

Go fish!

 

Hey sgtWookie,

I connected the output bk 2 da inverting terminal and then i gt a clean sine wave. Then i checked the juction of the two diodes and there is almost a constant signal of -7.42v and pretty much a similar thing at the bases of the two transistors...i dont understand wht tht is.....

Yes i have lukd evry whr for the model as well and cudnt find it but i will try it out on the breadboard directly,,the only thing im worried about is the slew rate is too slow(2v/us)...so it may nt work fr me as well

 

OK, if the junction of the two diodes is -7.2v, then the output of U3 is not connected to that junction.

You need to have the output of U3 connected to that junction, as well as to the inverting input.

I don't know how you can be measuring -7.2v there, as you don't show ANY negative supply on the schematic.

Unless, of course, your actual circuit varies greatly from the schematic.

 

Hi again,
yes now i have connected the output of U3 to the diode junction as well and i get a half sine wave that is clipped at the top at the output of U3,,,at the junction of diodes and at the base of transistors as well... actual circuit is very much the same as te schematic and the negative supply is Vee in the schematic.

 

So it is! I completely forgot about that part.

OK, measure the voltage at the junction of D1/R5/base Q1. Then measure at the emitter of Q1.

 

D2 could be burned open, Q1 could be unconnected or burned open - they would both cause the symptom you're seeing.

 

Hello,,,does any1 knw if an L272m works on +/- 12V or is it only +12 and GND..im confused from the datasheet
Thanks

 

Look at Fairchilds' datasheet. +12v/-12v is fine.

 

I guess you gave up on the original circuit?

 

Oh, if you're just going to use one channel of the L272, connect it's unused +input to ground, and it's output to the -input.

 

Thanks...im still working on the original cct 2..but having no success...i realised yeaterday that my Q1 and Q2 were blown...so have 2 replace them with new ones now...

meanwhile trying 2 see if a power amp can help....

 

Hey another new problem im having now...i put in new transistors Q1 and Q2 and now the power supply is shorting....is it because may b the two transistors r on at the same time .....Thanks for the help!!

 

Be certain that you have installed the transistors correctly. View the datasheet for each transistor.

Check to make sure that neither D1 nor D2 are shorted.

Check to make sure that the pull-up/pull-down resistors next to Q1 and Q2 are not open.

 

ive redone the whole circuit and im sure that evrything is connected fine.......it just doesnt wrk :-((