Op amp low pass active filter

[john.nash]

Hello,

The circuit attached is a low pass active filter, using a differential amplifier, but I don't know how it works. The inputs are the same signal, so I don't know how to obtain the gain.

Thanks in advance.

 

[godfreyl]

It acts more like a bandpass filter.

Mid-band gain is close to 470K/10K = 47
Low frequency rolloff is at 1 / (2*pi*10K*0.1uF) = about 160Hz
High frequency rollof is at 1 / (2*pi*470K*470pF) = about 650Hz

P.S. "about" because I don't have a calculator handy.

 

[john.nash]

Thanks godfreyl, but I simulated this circuit in LTSpice (without the input capacitors) and I obtained 5V at the output. The input was a sine signal of 1 V of peak and 600 Hz. So I don't understand what's happening.

 

[FvM]

No idea what you did exactly. Why don't you simply post the LTspice simulaton file?

1V input will overload the circuit.

 

[john.nash]

This is the simulation

 

[FvM]

You are driving the circuit with common mode input, not differentially. Ideally the output signal would be zero.

Practically there will be a certain output signal due to finite amplifier gain and common mode rejection ratio.

You should better refer to the original circuit with AC decoupling, otherwise the amplifier can be easily saturated by common mode signals and offset voltage.

 

[john.nash]

But the input 1 and 2 is the same, that is what I don't understand, if it is a differential amplifier how can be the signal amplified if the difference is zero. I put the capacitors and it's the same.

 

[FvM]

What do you want to achieve? To utilize the circuit as amplifier, connect a differential input. Your setup is just a common mode rejection test.

As said the simulation circuit should have zero signal output. What do you see?

 

[john.nash]

I see 5V in the output.

 

[FvM]

You mean 5V DC, that's expectable.

 

[john.nash]

The circuit is part of a receiver, the signal is demodulated with a mixer and is about 600 Hz. That is the input of the circuit (1 and 2). The manufacturer says that it is an amplifier.

 

[FvM]

Yes, it has gain of 47 as said in post #2. To use it as an amplifier the signal would be connected differentially, not as you did in the post #5 circuit.

By design, the circuit has an an output bias of 5V. That can't be avoided for a single supply amplifier.

 

[godfreyl]

...if it is a differential amplifier how can be the signal amplified if the difference is zero.

It can't, as youv'e found out. The input signal has to be applied between the two inputs.

If the mixer only has a single output, connect that to one of the amplifier's inputs, and connect the other input to ground.