problem with amplifier assignment

Thread Starter

newbie2019

Joined Apr 5, 2019
95
Our instructor gave us this circuit to figure out what is wrong.
I think it looks okay but must be something wrong.Could anyone point
me in the right direction?

much thx
 

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MisterBill2

Joined Jan 23, 2018
5,804
Our instructor gave us this circuit to figure out what is wrong.
I think it looks okay but must be something wrong.Could anyone point
me in the right direction?

much thx
The obvious problem is that I am unable to open the file to see the circuit. Are you able to post it as a PDF or a JPG file? I can open those very well.
 

MisterBill2

Joined Jan 23, 2018
5,804
Its an LTC Spice Simulator file.

Regards, Dana.
I thought it was, and I don't have any program to open spice files because i don't use spice. Thus I am unable to open it. On another thread somebody was decent enough to open a simulator file and post the image so that I could see it. And I just minutes ago had a go-round with some malware and so I am not about to invite some program claims to let me view spice files.
 

MisterBill2

Joined Jan 23, 2018
5,804
Will R3 provide enough base drive current? And it seems like R4 should have a high value bypass capacitor. Also, the use of T1 and T2 to develop forward bias is not cost effective. And there may be a bit of crossover distortion.
 

Thread Starter

newbie2019

Joined Apr 5, 2019
95
I assumed that the designer used T1 & T2 so that they would match T3 & T4, but I get it regarding not being cost-effective.

When I ran the simulation, the output power seemed very low.
 

Alec_t

Joined Sep 17, 2013
11,169
When I ran the simulation, the output power seemed very low.
With a 12V supply the maximum peak-to-peak output swing is 12V in an ideal amplifier. A practical amp will have a lower output swing. If you assume, say, 80% of the ideal value and halve that to give the peak amplitude of a sine-wave reaching the load, you can calculate the RMS output voltage and hence the maximum power in the 16 Ohm load. What do you get? How does that compare with the simulation result?
 

Thread Starter

newbie2019

Joined Apr 5, 2019
95
With a 12V supply the maximum peak-to-peak output swing is 12V in an ideal amplifier. A practical amp will have a lower output swing. If you assume, say, 80% of the ideal value and halve that to give the peak amplitude of a sine-wave reaching the load, you can calculate the RMS output voltage and hence the maximum power in the 16 Ohm load. What do you get? How does that compare with the simulation result?
The computed power out (rms) is 1.44 watts. The simulation result is 750 milliwatts and the curve is distorted on the peaks.
 
Last edited:

Alec_t

Joined Sep 17, 2013
11,169
The computed power out (rms) is 1.44 watts.
You forgot the √2 factor to get from peak voltage to RMS.
By my calculation, 80% of 12V = 9.6V pk-pk = 4.8V peak = 4.8V/√2 RMS = 3.39V RMS at the capacitor input. Hence max output power = (3.39^2)/16 = 0.72W assuming no loss in the capacitor. That's pretty close to the sim result.
Adjusting the input voltage amplitude and the base bias point of the input transistor should reduce the distortion of the peaks.
 

Thread Starter

newbie2019

Joined Apr 5, 2019
95
You forgot the √2 factor to get from peak voltage to RMS.
By my calculation, 80% of 12V = 9.6V pk-pk = 4.8V peak = 4.8V/√2 RMS = 3.39V RMS at the capacitor input. Hence max output power = (3.39^2)/16 = 0.72W assuming no loss in the capacitor. That's pretty close to the sim result.
Adjusting the input voltage amplitude and the base bias point of the input transistor should reduce the distortion of the peaks.

Thanks Alec_t and others for your help!
 

Thread Starter

newbie2019

Joined Apr 5, 2019
95
Sorry I need to bug you again. I noticed on the simulator graph that the positive
peak is +2.52 volts while the negative peak is -3.32 volts, difference of
0.80 volts. Not sure what is causing that.

The center point between the emitter resistors is 5.29 volts but I guess it
should be 6 volts?
 

Thread Starter

newbie2019

Joined Apr 5, 2019
95
OK. Is that causing the positive and negative peaks to be different?
I changed the Q3 bias resistors up and down in various combinations and of course the
output changed. The problem is that the output has a positive peak of 1.6 volts and a
negative peak of -2.0 volts. The THD is a horrible 6.85%.

I uploaded the current simulator schematic (revised).
 

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