Trouble prototyping an AB amplifier

Status
Not open for further replies.

kilwan

Newbie level 1
Joined
Jul 14, 2013
Messages
1
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Activity points
14
I was given a project in college to calculate, simulate and prototype an AB amplifier. My calculations match the multisim simulation. Now the last step I needed to do was prototyping the circuit. I made the circuit on a breadboard and went to the college lab.

So here is the circuit:
**broken link removed**

And here is the breadboard:
**broken link removed**
**broken link removed**

I should have gotten something like Vce1=15V and Vce2=15V
I got Vce1= 9V and can't really remember Vce2.
I used two DC sources that have (+),(GND) and (-) ports. For the input I used a function generator with (+) and (-) ports. Now I didn't even get to adding the function generator since my DC measurements weren't correct, so there really was no point in trying to do AC measurements. I hoped you guys had some tips on what to do, since I'm at a total loss here. I checked all of the connection, all of the resistors can handle the power .... Maybe I created a short somewhere ? Maybe I hooked up the DC source wrong to get the negative voltage I need ?

Would really appreciate some input
 

Hello Kilwan
have you tried connecting the load resistor R3 to the output via a decoupling capacitor ? most probably your circuit is drawing much Quiescent current
also test voltage drop on both diodes and BE of the transistors , the idea of diode biasing is that those voltage drops are identical , if not , try with other diodes
also the signal input should be connected to the circuit via decoupling capacitor
hope this helps
Regards
 

You have an imbalance in the idling current of the two transistors, or they are biased on too much. Try to change the circuit as below and monitor the voltage drop over R1/2 while adjusting R6. They should be the same. In this case, 15.5 mA is flowing through both (with no signal input). You don't want this current too high as to overheat your transistors. Try to keep it under 25 mA. R1/2 will also help with thermal stability and to reduce distortion. The transistors need to go on a heat sink if you are going to test full-power output of your amp. Furthermore, D1/2 should preferably be thermally coupled to their respective transistor for thermal compensation.
 

Attachments

  • amp_AB.gif
    43.9 KB · Views: 109
  • amp_AB_dist.gif
    43.6 KB · Views: 115
Last edited:

Status
Not open for further replies.
Cookies are required to use this site. You must accept them to continue using the site. Learn more…