Discussion in 'General Electronics Chat' started by xinyi9393, Jun 19, 2013.

1. xinyi9393 Thread Starter New Member

Jun 19, 2013
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may i know in this circuit, the amplifier is inverting or non-inverting? i'm very confusing about the input. pin 2 &3 both does have + input voltage.

and what is the uses for R4 1M ohms? why it's so big?

2. Jony130 AAC Fanatic!

Feb 17, 2009
3,939
1,089
LM741 in this circuit work just like a inverting amplifier with voltage AC gain
Av = R4/R1. And R2 and R3 provide proper bias point needed in this circuit becaoues of single supply use in this circuit.

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3. xinyi9393 Thread Starter New Member

Jun 19, 2013
3
0
thank you,
but why the power gain Av is so big?
Av= -(1M/12k)
= -83.33
and can i know one more thing, C1 is used to filter the input signal?
Appreciate.

4. Jony130 AAC Fanatic!

Feb 17, 2009
3,939
1,089
Maybe input voltage is so small that we need such a "large" voltage gain.
Yes C1 and R1 form a hight pass filter. But the main C1 function is to "block" any DC voltage present at the input. We don't want to gain any DC voltage.

5. xinyi9393 Thread Starter New Member

Jun 19, 2013
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but my input voltage is 9v, voltage after pass through R2 still have 2.63V.
so the Av = -83.33x 2.63V= -220V am i correct?

thanks~

6. #12 Expert

Nov 30, 2010
16,031
6,545
Yes, your math is correct and your reasoning is correct. If C1 was a wire instead of a capacitor, the amplifier would attempt to multiply 2.63 volts times 83.33. The resulting answer would be useless.

7. Veracohr Well-Known Member

Jan 3, 2011
540
75
It's not going to amplify 2.63V. It will amplify (+Input) - (-Input). If you have 2.63V on the +Input, it will amplify 2.63-signal. I'm guessing the AC signal voltage of the piezo will be much less than 2.63.

8. Ron H AAC Fanatic!

Apr 14, 2005
7,050
655
Due to feedback, the DC voltage on pin 2 will be identical to the voltage on pin 3 (2.53V). Due to the fact that C1 is infinite impedance at DC, the DC output voltage will also be 2.53V, plus the input bias current multiplied by the value of R4. The typical bias current is 80nA, so the DC output voltage will be typically be ≈2.61v. It could be a little lower, or as high as 3V.
The AC gain to the piezo input signal will be -83.3, as mentioned. However, piezo elements tend to be high impedance, so the gain may be less.