modified 8 years ago

Suggestions please

03:47:11

A ~ 40dB
Q = (945uA, 6.12V)

So, I need to drive this circuit with 10mV at the input. There appears to be a little distortion near to the positive peak of the output waveform. Can anyone make a suggestion as to what exactly causes this? 

published 8 years ago

hurz

8 years ago

Far to much gain. Lower it to about 20dB check this http://everycircuit.com/circuit/6519636257669120

crake

8 years ago

Unfortunately a minimum of 40dB is required.

crake

8 years ago

I may try this using real hardware and compare the difference

crake

8 years ago

http://everycircuit.com/circuit/5094669188071424

hurz

8 years ago

No chance with a single stage transistor amplifier and low THD

hurz

8 years ago

Use two stages or use an OpAmp

crake

8 years ago

Thank you, Hurz. In other circumstances I'd build a two stage. My prof wants us to design a single stage amp with a minimum gain of 40dB and a lower cutoff frequency of less than 100Hz. I was just curious if anyone knew the reason for this type of distortion.

thebugger

8 years ago

If you don't have a requirement for input voltage, the amp will behave better with 1mV input. Also, if you're not required to use a transistor, you can always make a better one with an op amp

2ctiby

8 years ago

Would something like this be of any help http://everycircuit.com/circuit/5755612536504320

thebugger

8 years ago

The gain depends very much from the transistor hFe and will be unstable this way. Use an op amp if your requirements allow for it.

hurz

8 years ago

HFe doesn't matter a lot. Check this with hfe 10 instead original 100 http://everycircuit.com/circuit/4970087982563328 the distortion is cause by a to high Vbe voltage swing. Keep it below <10mV

crake

8 years ago

I'd prefer to keep the input at or below 5mV but may just need to attenuate the 10mV input signal just a little and boost the gain to compensate for the reduction in output. If possible I would certainly use something other than a single stage, but this is what my professor want's. Other than me being picky about the distortion it will probably be fine considering it meets his other requirements.

thebugger

8 years ago

I was talking specifically about the example he made. The hfe there has a significant impact on the gain.

thebugger

8 years ago

What's Q mate? I saw you need something you call Q to be aroubd 1mA at 6.12V

thebugger

8 years ago

Anyway, this might do the trick http://everycircuit.com/circuit/5712784062152704

crake

8 years ago

Q is the quiescent point.

crake

8 years ago

Thank you all for your input btw

crake

8 years ago

Here are the full specs: input is 10mV, Vcc = Vee = 5V, Gain >= 40dB, F lower cutoff <= 100Hz, F upper cutoff >= 100k, Re coupling cap = 470uF, Rload bypass = 10uF, Rload = 100k, Rinput = 50.

thebugger

8 years ago

There's something wrong with the requirements. The emitter capacitor is way to big for a cutoff frequency of 100Hz. Do you mean 10Hz perhaps?

crake

8 years ago

The lab sheet says 470uF and lower cutoff below 100Hz. I may be wrong about the input. I'll have to ask him. He may have meant 10mV peak to peak

hurz

8 years ago

Buggzy you mean 470uF is good for 0.34Hz! Why only 10Hz, is this a buggzy estimation. Please use a calculator and formulas instead. Thanx.

hurz

8 years ago

The Hfe, as I showed for his circuit, is NOT significant. Why are you telling him it is if I proofed its not?

hurz

8 years ago

@crake, again, a single transistor amplifier with 40dB voltage gain must have big disadvatages on other properties. You must specify exactly for which input and output fingerprint you are designing such a amplifier. Current, voltage, bandwidth, distortion.

thebugger

8 years ago

Hurz, it's just a school project. It's not supposed to be super linear or anything, just to work inside these specifications. And what I meant by the frequency response was that 470uF@100Hz correspond to an emitter resistor of 3.3ohm. That's not even worth bypassing. So perhaps I chose 10Hz as a more realistic rolloff frequency where the enitter resistor will be 33ohm. I'd take it even further at 5Hz for instance, but not any lower

hurz

8 years ago

Thebugger, you see more information then me.

add comment in editor