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R 3.3k R 3.3k 10n: - OUT OS1 - OUT OS1 - OUT OS1

1) The document describes an experiment to realize a band-pass filter (BPF) and low-pass filter (LPF) using a Tow-Thomas biquad circuit with operational amplifiers. 2) For the BPF, the input signal frequency is varied from 100Hz to 10KHz and the output voltage is measured, which should be maximum at 5V. 3) For the LPF, the input signal frequency is varied from 10Hz to 10KHz and the output voltage is measured, which should also be maximum at 5V. 4) Questions are asked to calculate filter characteristics from the circuit and compare to measured values, and derive the transfer functions.

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95 views3 pages

R 3.3k R 3.3k 10n: - OUT OS1 - OUT OS1 - OUT OS1

1) The document describes an experiment to realize a band-pass filter (BPF) and low-pass filter (LPF) using a Tow-Thomas biquad circuit with operational amplifiers. 2) For the BPF, the input signal frequency is varied from 100Hz to 10KHz and the output voltage is measured, which should be maximum at 5V. 3) For the LPF, the input signal frequency is varied from 10Hz to 10KHz and the output voltage is measured, which should also be maximum at 5V. 4) Questions are asked to calculate filter characteristics from the circuit and compare to measured values, and derive the transfer functions.

Uploaded by

kcb26
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
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Experiment 9 Realization of Band-pass and low-pass filter using Tow-Thomas biquad circuit (Experimental) Reference Chapter 16 p.

. 1297-1298 (Sedra and Smith, 6th edition) Introduction Electronic filters are important building blocks in communication and instrumentation systems. They are characterized by their signal transmission properties. For example, a low-pass filter (LPF) is the one that blocks high-frequency component of the signal while allows the low-frequency part to pass. Similarly, a band-pass filter (BPF) blocks low-and high-frequency component but let a band to pass. So, filters are classified into different types based on relative location of their pass-band and stop-band(s). In this lab you would realize a BPF and an LPF using Tow-Thomas biquad circuit. Theory The circuit in Fig. 1 is a second order (i.e. the frequency response function has a second-order denominator) BPF. It uses 3 Opamps and a few RC elements.
R R 3.3k 3.3k

C
10n
4 V2 OS1 1 6 5

C 10n
V-

r r 10k 10k vi r
3

OUT OS2

uA741

OS1

V+

OUT

V-

U1 +

3.3k
3

6 5

uA741

10k
OS1 1 6 5

U2 +

OS2

vo

uA741 OUT U3 +

V+

V+ 7

OS2

Fig.1-A band-pass filter The transfer function for this circuit is given by the following expression.

The mid-band gain is unity and the mid-band frequency is given by f0 = 1/2RC. The circuit in Fig. 2 is a LPF. Compared to BPF, the location of input v i has changed. Addtionally, resistor r is replaced by R.

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R R 3.3k 3.3k

C
10n
4 V2 OS1 1 6 5

C 10n
V-

r r 10k
3

OUT OS2

uA741

OS1

V+

OUT

V-

U1 +

3.3k
3

6 5

uA741

10k
OS1 1 6 5

Fig. 2- A low-pass filter The transfer function for this LPF is given by the following expression.

The low-frequency gain is unity and the 3dB frequency if f0 = 1/2RC. Experimental Procedure The work in this laboratory session uses 3 Opamps (LM741) and various components of R and C. The pin-out for LM 741 is given in Fig. 3.
offset 1 inverting 2 non-inverting 3 -Vcc 4 5 offset + 6 7 OUT 8 +Vcc

+ -Vcc

+ +Vcc

Fig. 3 (The pin configuration for LM 741)


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vi

V+

vo 3.3k

U2 +

OS2

uA741 OUT U3 OS2

V+

We use the power supply to get Vcc =10V. Pin 7 takes +10V whereas, pin 4 takes -10V. Caution! Once in a while LM 741 goes bad in the middle of the experiment. Be prepared to change it and throw away the bad one.

Band-pass filter Assemble the circuit in Fig. 1. For the circuit in Fig. 1 use the sinusoidal
Vin (p-p) =5V. Vary the frequency between 100Hz and 10 KHz and note the output voltage Vo (p-p). Note that the maximum value of Vo should be around 5V (p-p) as this is a unity gain filter. If you fail to get the desired result after assembling the components correctly, it is possible that one of the LM 741 is bad. One way to check if the Opamp is working correctly is to make a voltage follower. It is shown in Fig. 4. This works only after the Opamp is given the supply voltage (i.e. pin 7 has+10V and pin 4 has -10V).

V in

A741 +

V out

Fig. 4 Voltage Follower (Vin = Vout)

Low-pass filter Modify the circuit in Fig. 1 to get Fig. 2. The entire circuit is same except
for the location where Vi is applied. There is another difference: now Vi is applied through 3.3K resistor. For this circuit use sinusoidal Vin (p-p) =5V. Vary the frequency between 10Hz and 10 KHz and note the output voltage Vo (p-p). Note that the maximum value of Vo should be around 5V (p-p) as this is a unity gain filter. Questions 1. 2. 3. 4. Calculate the mid-band frequency for BPF and compare it to your measured value. Calculate the 3 dB point for LPF and compare it to your measured value. Derive the transfer function for BPF (Eqn. 1). Derive the transfer function for LPF (Eqn. 2).

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