Trip 4. Operational Amplifiers
This fourth trip introduces a fundamental building block in analog and digital electronic circuit design: the operational amplifier (Op Amp). After the ideal Op Amp is presented, the two main modes of operation are described: the inverting and the non-inverting amplifier.Several details related to the finite open loop gain, slew rate, output saturation and DC imperfections are illustrated in theory and through experiments as well.
Please click to preview the experiments in this Trip: 1, 2, 3, 4, 5, 6.
![1](javascript:ReplaceContentInContainer('content-exp',' <h3>Experiment 1. Signal Conditioning Op Amp I</h3>
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<img src="./images/Exp1SigCondI.jpg" width=" 333 " align=" right" style=" float:right; padding-right:10px; padding-bottom:5px;">
<p> The <b>Topnotch Meter</b> Generator has a limited output range from 0V to 2.048V. In some applications it is necessary to increase this range. For example a varactor diode (variable capacitance diode) requires a tuning voltage from 1V to 8V. To perform this increase in range an operational amplifier is used.
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<p> In this experiment the signal is just level shifted and inverted. No amplification is performed although with a few minor changes amplification can be obtained. The <b>Topnotch Meter</b> is used to monitor the input and the output of the operational amplifier and to prove its operation is according to what is expected from theory.
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![2](javascript:ReplaceContentInContainer('content-exp',' <h3>Experiment 2. Signal Conditioning Op Amp II</h3>
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<img src="./images/Exp2SigCondII.jpg" width=" 333 " align=" right" style=" float:right; padding-right:10px; padding-bottom:5px;">
<p> The need to expand the <b>Topnotch Meter</b> Generator range is combined with the need to buffer signals, and amplify with a positive or negative gain. Occasionaly an operational amplifier can be used as an ad-hoc power supply.
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<p> In this experiment the signal is level shifted, amplified and it is not inverted (the Op Amp is in non-inverting configuration). The amplification is combined with a superposition of a voltage derived from the power supply which ensures proper DC shift. Once again it is possible to use the <b>Topnotch Meter</b> to verify the behaviour of the Op Amp circuit and to demonstrate that theory and experiment are in agreement.
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![3](javascript:ReplaceContentInContainer('content-exp',' <h3>Experiment 3. Integrator Op Amp</h3>
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<img src="./images/Exp3IntegratorOpAmp.jpg" width=" 333 " align=" right" style=" float:right; padding-right:10px; padding-bottom:5px;">
<p> The goal of this experiment is to integrate the square wave generated in Experiment 2: Signal Conditioning Op Amp II. The integration is performed using a combination of an Op Amp and a capacitor properly configured.
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<p> In Experiment 2 the input from the <b>Topnotch Meter</b> Generator was a square wave and output of the Op Amp a level shifted, amplified square wave. In Experiment 3, the square wave at the output of the Op Amp is integrated using another Op Amp and a capacitor both in theory and in experimental form and transformed into a triangular wave. You can see this on the screen of the <b>Topnotch Meter</b>, in the image.
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![4](javascript:ReplaceContentInContainer('content-exp',' <h3>Experiment 4. Differentiator Op Amp</h3>
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<img src="./images/Exp4DiffOpAmp.jpg" width=" 333 " align=" right" style=" float:right; padding-right:10px; padding-bottom:5px;">
<p>The goal of this experiment is to differentiate the triangular wave generated in Experiment 3: Integrator Op Amp.
The differentiation is performed using a combination of an Op Amp and a capacitor properly configured.
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<p> In Experiment 3 the output of the integrator Op Amp was a triangular wave. In Experiment 4 the triangular wave at the output of the Op Amp is differentiated using another Op Amp and a capacitor both in theory and in experimental form and transformed into a square wave. Remember, integrating and differentiating a square wave should lead you to a square wave. You can see this on the screen of the <b>Topnotch Meter</b>, in the image.
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![5](javascript:ReplaceContentInContainer('content-exp',' <h3>Experiment 5. Lossless Low-pass Filter</h3>
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<img src="./images/Exp5BufferedLowFreqAmp.jpg" width=" 333 " align=" right" style=" float:right; padding-right:10px; padding-bottom:5px;">
<p> The goal of this experiment is to compare the effect of loading on a RC Low-pass filter with the effect of loading on a buffered RC Low-pass filter. Buffering is a way to isolate a signal by feeding it as an input to an Op Amp and replicating it to the output of an Op Amp. This process makes the replica of the signal less sensitive to loading (connecting a load resistor at the output of an Op Amp).
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<p> In this experiment, it is shown how the output changes when an RC Low-pass filter is loaded with a resistor, and when an RC Low-pass filter buffered by an Op Amp is loaded with a resistor. Clearly, the output decreases in the unbuffered circuit and is almost intact in the buffered circuit.
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![6](javascript:ReplaceContentInContainer('content-exp',' <h3>Experiment 6. DC Imperfections</h3>
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<img src="./images/Exp6OffsetCurrent.jpg" width=" 333 " align=" right" style=" float:right; padding-right:10px; padding-bottom:5px;">
<p> The goal of this experiment is to measure two Op Amp DC imperfections namely the input bias current and the offset voltage. A real Op Amp is not an ideal circuit. For example, the current going into the input terminals, called input bias current, should ideally be zero. In reality this current is small but not zero.
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<p> Another DC imperfection is the offset voltage. Due to mismatches if the two input terminals of the Op Amp are tied together and to ground, the output is not zero. Although, theoretically the output should be a multiplication of gain by zero, the output is a multiplication of gain by offset voltage which is small but not null.
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Experiment 1. Signal Conditioning Op Amp I
The Topnotch Meter Generator has a limited output range from 0V to 2.048V. In some applications it is necessary to increase this range. For example a varactor diode (variable capacitance diode) requires a tuning voltage from 1V to 8V. To perform this increase in range an operational amplifier is used.
In this experiment the signal is just level shifted and inverted. No amplification is performed although with a few minor changes amplification can be obtained. The Topnotch Meter is used to monitor the input and the output of the operational amplifier and to prove its operation is according to what is expected from theory.