Design a circuit to drive an 8Ω, 0.25W speaker from the output of a function
generator and show the simulation to your instructor. Here are the design specs:


• You may not use more than one supply voltage, i.e., you should design a single supply circuit. If your circuit
needs more than one voltage, e.g., for biasing, you should generate it using resistor dividers, zener voltage
sources, and the like.
• Supply voltage shall be 3V. This is the voltage of 2 AA batteries in series.
• The test input is a 300mV peak-to-peak, 1kHz zero-offset sine-wave.
• Multiple stages are allowed. Example stages include CE amp and emitter follower.
• To get full points for this exercise, you’ll need to implement a small-signal voltage gain of about 7 and have no
noticeable distortion on the speaker waveform.
• Only BJTs, diodes, and passive components are allowed.

I'm trying to get a 7 Small signal voltage but can only reach 4, how can I make it 7?

Thanks a lot for you responses!!

 

Why is your first stage common-base and not the more usual common emitter?

 

2.1V peak to peak from a 3V supply is going to be a bit of a challenge.
The usuaL complementary emitter follower output stagewill only give 1.6V.

 

Why is your first stage common-base and not the more usual common emitter?

Thank you for your response, sir. I was struggling with voltage biasing in common emitter configuration, although I have a high voltage gain the downside is the clipping in the negative part of the wave. Due to slight distortions not being possible, I chose not to use the said configuration. With the properties of a common base configuration of having a low input impedance as opposed to a common emitter with a high input impedance to be matched with a load (speaker) with a low impedance, I figured I had to match their impedances in order for them to have less to no noise. Sir, what do you suggest I do with my configurations?

 

2.1V peak to peak from a 3V supply is going to be a bit of a challenge.
The usuaL complementary emitter follower output stagewill only give 1.6V.

I have indeed experienced problems with amplifying my signal more than 1.2Vpp, do I stick with my configurations or try to use common emitter configuration? If so, what should I take note in biasing the resistors and deciding the values of the other resistors?

 

what do you suggest I do with my configurations?

You could use a first stage common-emitter stage with an emitter resistance to give the desired gain of 7, driving the Darlington output you already have.

 

You could use a first stage common-emitter stage with an emitter resistance to give the desired gain of 7, driving the Darlington output you already have.

I replaced it with a common emitter, but whenever my output voltage is greater than 600mV the transistors go into saturation thus clipping the negative part of the wave.

 

R6 is very high compared to R5. Hence it has little effect.
Remove R6.
Increase value of R5.

Edit:
R7 is very high compared to R6. Hence it has little effect.
Remove R7.
Increase value of R6.

Note: Don't change your component designators from one schematic to the next.

 

R6 is very high compared to R5. Hence it has little effect.
Remove R6.
Increase value of R5.

Edit:
R7 is very high compared to R6. Hence it has little effect.
Remove R7.
Increase value of R6.

Note: Don't change your component designators from one schematic to the next.

I tried to increase R6 and remove R7, it results to the graph to clip at the bottom.

I tried it both on Common Base and Common Emitter, the results are still the same.

 

Have you calculated the resistor values you need to bias the transistor properly so the clipping will be symmetrical, giving the maximum undistorted output voltage?
Random selection of component values is not a design is done.

If you don't know how to do that, than look up common-emitter amplifier with feedback.