Building a difference amplifier with a TL081CN

Discussion in 'General Electronics Chat' started by bucky54321, Feb 16, 2006.

1. bucky54321 Thread Starter New Member

Feb 16, 2006
5
0
Hello,
I want to build a difference amplifier using the operational amplifier TL081CN. I use 100 Kohm Resistors for the circuit and my supply is +Vcc=+10V and -Vcc=0V. The result is that when i try to make this circuit work with the same voltage in the inputs i get Vout ≈ 1.3V at the output instead of the normal Vout=0V.Moreover when I apply different voltages at the input i don't get their difference but something else. My query is if the specific IC cannot work as a difference amplifier or if i do something wrong with the supply voltages. I have also tried with voltages +Vcc=+5V and -Vcc=-5V but the circuit still didin't work.

2. jatulm New Member

Feb 13, 2006
7
0
Hi.

I think It would be very helpful to see the schematic of your circuit, since y don't see how you implment the diff amp.. generally diff amps are not implemented with opams but with transistors directly.

applying the same voltage to the inputs of the opamp, won't necessarily drop a 0v, nor won't drop the difference. feedback of the opamp in your circuit is also very important for this...

now, as I said before, seeing your schematic would be very very helpful

regards,

3. Dcrunkilton E-book Co-ordinator

Jul 31, 2004
416
11
We assume that you are trying the first "one op-amp" circuit here . That you are using four equal value resistors. The +5V -5V power supply will be less troublesome when you first start out, because the (0V) referene will be centered between the power supply rails. Check that the center point, common point of your +5V -5V PS is connected to your ground. That is, any inputs, or outputs are referenced to this ground. That means that if you connect +3V to one input, the (+) side goes to the resistor (input), the (-) side goes to the ground, the center point of the PS. Same for the other input, say +2 V, to the other input resistor. The output of the OPA is measured with respect to ground.

In case of problems.
1. measure the +Vcc and -VCC pins at the OPA package with respect to ground to verify +5V, -5V at the OPA
2. It would not hurt to measure the two input voltages at the input resistors with respect to ground, +3V, +2V in our example.
3. A good functioning OPA with a feedback resistor from OPA output to (-) OPA input pin will show zero volts measured between the two OPA input pins (+), (-). If this is not true, the OPA is bad or the feedback resistor is open. Another way of stating this is that both of these pins measure the same voltage with respect to ground.

One more note on the OPA P/S. It could be as simple as two 9V batteries connected in series, with the series connection used as ground. There are three wires from this battery P/S : (+9) , (-9), (ground).

4. Dave Retired Moderator

Nov 17, 2003
6,960
145
Moved Topic.

This thread is better suited to the General Electronics forum. If the opening poster feels that this topic is better suited in the original forum, please PM me.

Thanks.

Dave

5. dragan733 Senior Member

Dec 12, 2004
152
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The cause is maybe from the tolerance of the resistors. See that the resistors are with tolerance: 0,1%.
The second cause is maybe from damaged TL081, change it with new
The third cause is maybe from the offset voltage drift of the TL081. Try to decrease this drift with a potentiometer placed between the pins 1 and 5 with the potentiometer's middle to -Vcc