Some applications of the operational amplifier. Scheme, description

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Some Applications of the Type 741 Operational Amplifier. Encyclopedia of Radio Electronics and Electrical Engineering

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The widely used operational amplifier (op-amp) of the 741(140UD7) type has a large common-mode noise suppression ratio, which makes it possible to implement a variable-gain amplifier and even an analog switch on its basis at low cost. The gain of the op-amp can be easily adjusted by changing the ratio between the signals arriving at its inputs. In the case of equal signals at the inputs, they are suppressed as common mode noise, and the output voltage is zero. When the signal at one input is greater than at the other, it is amplified.

In the variable gain amplifier circuit (a), the resistors R1 and R2 are chosen as usual, based on the given input resistance (in this case f1/2) and the gain (-R2/R1). By changing the resistance of the feedback resistor R2, you can achieve maximum attenuation of the signal at the maximum resistance of the resistor R3 (R3 = R2), which regulates the gain. In practice, when adjusting the control resistor from maximum to zero, the gain changes from zero to -R2/R1 without shifting the DC output level.

Some Applications of the Type 741 Operational Amplifier

This circuit can also be used as an analog switch (b). In this case, however, there are the following restrictions: mandatory capacitive coupling at the output, input signal swing no more than 1,2, and unity op amp gain (R1=R2=R3). Under these conditions, the circuit works like an ideal analog switch.

When a logic 1 level (2,4-4 V) is applied to the input of the digital control circuit, the transistor saturates and grounds the resistor R3, as a result of which the signal is attenuated by 70-90 dB. When the transistor is turned on, the potential at the non-inverting input of the op-amp becomes 0,6 V, causing a corresponding voltage of 0,6 V DC at the output, and therefore a transition capacitor is required.

When a logic zero (ground level) is received, the transistor turns off, as a result of which the non-inverting input of the op-amp is ungrounded, and the signal passes through the amplifier. If the positive voltage of the control signal is sufficient to open the base-collector junction, then positive spikes seep into the output signal.

Author: Beach McGraw-Hill Center of Excellence (Washington, DC); Publication: N. Bolshakov, rf.atnn.ru

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