need help to understand a circuit

[ziphlox]

hi everybody,
there is a circuit in the simulation software "proteus isis" which is supposed to control a servomotor, the circuit actually looks good and works just fine, but i just found some difficulties trying to understand the role of some components of it, such as the diodes D1, D2, D3 & D4.
i attached the circuit as an image to this thread. i hope you can help.
thanks in advance.

moderators note .... please dont upload such huge bmp files in future use compression software to jpg or gif 169k is much better than 1.8MB
Dave

 

[duke37]

The transistors Q1 and Q2 are presumably silicon devices and need 0.7V base/emitter to turn them on, therefore 1.4V is needed between the bases to eliminate a dead band.
The 1.4V is generated by supplying a current through two diodes which drop 0.7V each similarly to the transistors.

 

[BobK]

This is the standard arrangement for a push-pull amplifer. As Duke said, the diodes help mitagate a problem where neither transistor is turned on. This is know as crossover distortion.

Bob

 

[ziphlox]

thank you guys so much for the clarifications.
i would be very grateful if you can also explain to me the role of the little circuit which contains the zener diode D5 connected to the transistor Q7 and the resistance R3 and R6, i mean i know it's a current generator, but i just don't understand why it should be used in their and connected to the differential stage ( composed of Q5 & Q6 ) which i am not sure whether it exactly commands or is commended by the two transistors Q8 and Q9.
thank you so much for your time and help.

 

[(*steve*)]

I think if you google "long tailed pair" you'll get a good explanation.

I'm a bit rushed at the moment to explain.

 

[duke37]

As you say this is a current controller. The current is switched with Q5 and Q6 to define the current in the bases of Q8 and Q9.

The zener makes the current constant even if the supply voltage varies so the circuit can be used on a wide range of voltages.

 

[CDRIVE]

hi everybody,
there is a circuit in the simulation software "proteus isis" which is supposed to control a servomotor, >>>>>>

That is not a servo motor or servo motor control circuit.

Chris

 

[BobK]

Actually, I think it is, not for a typical hobby servo, but for a motor that incorporates a potentiometer for position feedback. Note the 5 leads, with 2 connected like power to an H-bridge and the other like a potentiometer that provides feedback to the differential amp.

Bob

 

[CDRIVE]

Bob, I think you're correct. After I posted that I opened my ISIS, which I rarely use, and took a peek at the available motor models. I should have loaded them to read the properties. I didn't.

Statement retracted.

Chris

 

[ziphlox]

you have been really so helpful guys, thank you from the bottom of my heart.

 

[CDRIVE]

I'm curious. Is your circuit supplied in an 'Examples' folder within ISIS?

Chris

 

[ziphlox]

yeah, indeed it is.
and it's Under the name SERVO.DSN

 

[CDRIVE]

Thanks for the reply.

Chris