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Wednesday, October 12, 2011



Aim:   To verify the different circuits of a function generator using op-amp

  1. Function generator using 741 op-amp kit
  2. Dual trace oscilloscope
  3. digital Multimeter
  4. connecting wires


Function generator is a signal generator that produces various specific waveforms for test purposes over a wide range of frequencies. In laboratory type function generator generally one of the function (sine, triangle, etc.) is generated using dedicated chips or standard circuits and converts it in to required signal.

Sine wave oscillator (wein-bridge oscillator):

This is a basic wein bridge oscillator using op-amp. Diodes (4148) across feedback resistor are used to maintain constant output voltage. A 47 K dual potentiometer is used vary the frequency for limited range 2 K 10 turn trim pot helps to adjust the gain at high resolution capacitor 0.1F Couples the signal to the next section.

Zero crossing detector (sine square converter):

Fig 1.2 shows the zero crossing detectors using 741 op-amp. In this circuit 741 is connected to work as a comparator. Non-inverting input is connected to the voltage divider network. Sine wave is connected to the Inverting input of the op-amp. Voltage at Non-inverting terminals sets the reference level. When the input sine amplitude exceeds the voltage at Non-inverting terminal output switches to either positive or negative saturation, ultimately the output of the zero-crossing detector is a square wave.

Integrator (square to triangle converter):

Figure shows integrator-using op-amp. Square wave from the zero crossing detector is fed to the integrator using op-amp. RC time constant of the integrator has been chosen in such a way it is a small value compared to time period of the incoming square wave. As you knew the operation of integrator, the output of the integrator is a triangle wave we feed square wave input.

Switching circuit (function selection):

Circuit of the switching circuit is shown in figure this consists of four channel analog multiplexer (CD 4502), 2-pole 3-way switch and FET input quad op-amp IC TL 084, as shown in circuit this has three inputs namely A,B,C and one output Multiplexer has been connected in such a way when one of the inputs (A,B,C) can be selected with the help of switch. When we connect signals from sine oscillator, zero crossing detector and integrator (sine, square and triangle signals), we can select the desired signal and feed it to the output amplifier.


Figure shows the output amplifier used in this trainer. In this 741 op-amp is connected in Non-Inverting amplifier. The gain of the amplifier is set to the5. This represents the output amplifier in the commercial function generator. Signal selected from the switching circuit can be amplified by 5 times.


1.      Connect trainer to the mains and switch on the supply.
2.      Measure the output voltage of the regulated power supply i.e., 5V supply.
3.      Observe the output of the sine wave oscillator (wein bridge oscillator). If signal is not coming or distorted in shape adjust the gain trim pot until you get the good signal. Measure the signal frequency-using Oscilloscope.
4.      Observe the output of the zero crossing detector (square wave) by varying the input signal (sine wave is internally connected to the circuit) frequency.
5.      Observe the output of the Integrator (triangle) by varying the input signal frequency (square wave is internally connected to the circuit).
6.      Now connect sine, square, triangle signals to the switching circuit.
7.      Connect output of the switching circuit to the output amplifier. Select the desired signal-using switch.
8.      Observe the final output.

Circuit Diagram :


Find the charging time tc discharging time td and totals time period T from the output waveform.
7.      Verify these values with theoretical values and calculate the % of the duty cycle.
Where Tc = 0.69 (RB+RA) C
                                                          RA = R2+R1
     Td = 0.69 (RB C
T = Tc+Td
                                                                                             f = 1/T
% of DC


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Circuit Diagram:


Expected waveforms:




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