# Designing a Band Pass Amp: Op-Amp & TL071 Help

• cps.13
In summary, the author is seeking help with an electronics project and has decided to use an inverting op amp setup. He is also looking for help understanding active opamp filters. He has read that inverting amps are typically used for filters, and has calculated that he will need 2 of them. He is also looking to set the gain of the filter at 10db.
cps.13

## Homework Statement

I have an electronics project to design and build a band pass amp for my HNC. The only thing I have been given is it must use a uA741 or TL071 with an input imp greater than 1k and bandwidth between 200hz to 10khz. And gain must be 10db.

It is to be used in battery powered equipment.

## Homework Equations

Part of the report write up is that I need to justify my choice of chip and use of either inverting or non-inverting.

## The Attempt at a Solution

I have decided to use an inverting op amp setup as I can simply put a 1k resistor on the input and that is my input impedance sorted. Also makes calculating the gain easy!

I will use the TL071 as the output will not drop off at higher frequencies. The uA741 shows it will drop off at around 10kHz.

My problem is we have been taught nothing about a band pass amplifier/filter. So I am unsure if it makes any difference using a inverting/non-inverting setup?

I also intend to put a couple of variable resistors in before R1 and R2 to help get the gain as accurate as possible.

Any help is appreciated, if I am missing anything obvious could you point me in the right direction?

Thanks.

cps.13 said:

## Homework Statement

I have an electronics project to design and build a band pass amp for my HNC. The only thing I have been given is it must use a uA741 or TL071 with an input imp greater than 1k and bandwidth between 200hz to 10khz. And gain must be 10db.

It is to be used in battery powered equipment.

## Homework Equations

Part of the report write up is that I need to justify my choice of chip and use of either inverting or non-inverting.

## The Attempt at a Solution

I have decided to use an inverting op amp setup as I can simply put a 1k resistor on the input and that is my input impedance sorted. Also makes calculating the gain easy!

I will use the TL071 as the output will not drop off at higher frequencies. The uA741 shows it will drop off at around 10kHz.

My problem is we have been taught nothing about a band pass amplifier/filter. So I am unsure if it makes any difference using a inverting/non-inverting setup?

I also intend to put a couple of variable resistors in before R1 and R2 to help get the gain as accurate as possible.

Any help is appreciated, if I am missing anything obvious could you point me in the right direction?

Thanks.

What order is your filter going to be? What polynomial are you going to use and why? What reading have you done about active opamp filters?

Just off the top of my head without any research, I would use two inverting amps. Perhaps differentiator and a integretor in series. Set the RC constant at 200hz for the differentiator and set the RC constant for the low pass filter at 10Khz. Keep the first amp passive, make the second amp active.

Is your 10db gain a voltage gain or a power gain?

If trying to get a 10db gain in power, you will need your gain to be 10.
10^(db/10)= Power Gain.
10^(10/10)= 10
Or going backwards, 10 log 10= 10db

If trying to get a 10db gain in voltage, you will need your gain to be set at 3.16.
10^(db/20)= Voltage Gain.
10^(10/20)=3.16
Or going backwards, 20 log 3.16 = 10db

I'm not an expert on this, but perhaps a few of my comments will stear you in the right direction. You may be able to use just one amp to accomplish the same thing.

I don't think it makes a difference if you use a inverting or non-inverting as long as your transfer function meets your requirements. We generally used inverting amps for filters in school.

Last edited:
Just some food for thought...

Some times it pays to go back to the definition. Decibels are defined as:

dB=10Log(Po/Pi) Po=power out; Pi=power in.

When power is expressed as V2/R then: (where V2 means V squared)

Po=Vo2/RL and Pi= Vi2/Ri; when RL=Ri

dB=10log((Vo2/RL)/(Vi2/Ri)) since RL=RI the R's cancel and

dB=10log(Vo2/Vi2)=10log(Vo/Vi)2 since this a log function the square comes forward, and

dB=20log(Vo/Vi)

The are several variations of dB's, dBm in particular you should be familiar with.

To quote a well know author, "You should keep the original definition of the decibel firmly in
mind because it is of fundamental importance in many engineering applications."

You can design a simple bandpass filter with 1 op amp, 2 resistors and 2 capacitors. Total.

The transfer function would be T1s/(T1s+1)(T2s+1) with T1 corresponding to 200 Hz and T2 to 10 KHz.

If you don't know laplace transforms, just substitute s = jw in the above transfer function.

I would use a higher input resistor than 1K ohm. Like 10K.

Since you want the gain to be 10 dB = 3.16, think Rf/Ri = 3.16.

## 1. What is a band pass amplifier and how does it work?

A band pass amplifier is a type of electronic circuit that is designed to amplify a specific range of frequencies while attenuating all other frequencies. It works by using a combination of resistors, capacitors, and op-amps to create a frequency response curve that allows only the desired frequencies to pass through.

## 2. What is the role of an op-amp in a band pass amplifier?

An op-amp, or operational amplifier, is a key component in a band pass amplifier. It is used to amplify the input signal and provide the necessary gain for the desired frequency range. It also helps to maintain the stability and linearity of the amplifier's output.

## 3. Can any op-amp be used in a band pass amplifier circuit?

No, not all op-amps are suitable for use in a band pass amplifier circuit. It is important to select an op-amp with a high gain-bandwidth product and low input offset voltage for optimal performance. The TL071 is a popular choice for band pass amplifier circuits due to its low noise and high gain capabilities.

## 4. How do I choose the appropriate values for the resistors and capacitors in a band pass amplifier?

The values of the resistors and capacitors in a band pass amplifier circuit will depend on the desired frequency range and the specifications of the op-amp being used. It is important to calculate the cutoff frequencies and select appropriate values that will provide the desired gain and bandwidth. There are also online calculators and software programs available to assist with this process.

## 5. What are some common challenges in designing a band pass amplifier?

Some common challenges in designing a band pass amplifier include achieving the desired frequency response curve, maintaining stability and linearity, and minimizing noise and distortion. It is important to carefully select the components and design the circuit with precision to overcome these challenges.

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