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Help with Unity Gain Amplifier

by sailmike
Tags: amplifier, gain, unity
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AlephZero
#19
Apr8-14, 08:17 PM
Engineering
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HW Helper
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P: 6,957
You don't have to shift the amplifier output by 0.6V. You could shift the zero reference of the ADC by 0.6V to compensate.
sailmike
#20
Apr8-14, 10:23 PM
P: 26
Oh I know to use resistors to bias the diode. I just don't know where to connect the other end. I tried the connection shown in the attachment, which I know is wrong, since I'm getting the wrong output. I just realized that the diode is not forward biased in this circuit. Adding an Rc resistor changes the output voltage at Ve.

Thanks,
Mike
Attached Thumbnails
S and H Amplifier 3.jpg  
mjhilger
#21
Apr8-14, 11:13 PM
P: 93
Mike, I owe you an apology. I said collector and I meant emitter. The cathode attached at the emitter and a resistor from the anode to +5 top rail will forward bias the diode and your anode will be approximately 0.6 v above the emitter. But Alephzero also has a good suggestion of using the offset on the A/D reference circuit.

Welcome to design engineering! You always have choices and I have struggled at times with two good solutions trying to determine which is the best. Ultimately you must decide a direction and implement.

Again, I'm really sorry for stating the wrong leg.
mitch
sailmike
#22
Apr9-14, 12:03 AM
P: 26
Oh no problem, we all make mistakes. We wouldn't learn anything if we didn't.

That makes better sense. Right now it's cutoff around zero. The negative portion of the signal is cutoff.

Thanks,
Mike
sailmike
#23
Apr10-14, 02:37 AM
P: 26
Alright, here's what the circuit looks like now. The results are wrong though. I can't seem to get a 0.6V difference between the cathode and anode of the diode, perhaps because of the specific diode I'm using in the simulation. Also attached is the output graph. The negative side is partly clipped.

If I was to offset the reference voltage, how would I do that? I'm allowed to use a DAC chip and that has a Vref input. Can I use that input to offset the reference voltage? If I go that route, it may be a good idea to use a potentiometer in place of a resistor so I can adjust the reference voltage to match the offset more precisely.

Thanks,
Mike
Attached Thumbnails
S and H Amplifier 4.jpg   S and H Amplifier Output Graph.jpg  


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