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—university:courses:electronics:electronics-lab-variable-gain-amplifier [26 Mar 2018 12:09] – add Variable Gain Inverting Amplifier using potetiometer Antoniu Miclaus
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 +====== Activity: Variable Gain Amplifiers ======
 +
 +===== Objective =====
 +
 +In this laboratory we continue our discussion on operational amplifiers (see the previous lab here: [[university:courses:electronics:electronics-lab-1|Activity 1. Simple Op Amps]]) focusing on variable gain / voltage-controlled amplifiers.
 +
 +Most operational amplifier or op-amp circuits have a fixed level of gain. However it is often useful to be able to vary the gain. This can be done simply by using a potentiometer on the output of a fixed gain op-amp circuit, but sometimes it may be more useful to vary the actual gain of the amplifier circuit itself.
 +
 +A variable-gain or voltage-controlled amplifier is an electronic amplifier that varies its gain depending on a control voltage. This type of circuit has many applications, including audio level compression, synthesizers and amplitude modulation. It can be realized by first creating a voltage-controlled resistor, which is used to set the amplifier gain. The voltage-controlled resistor is one of the numerous interesting circuit elements that can be produced by using a transistor with simple biasing. Another approach is to use potentiometers to vary the value of the resistors that set the gain of the amplifier.
 +
 +===== Materials =====
 +
 +ADALM2000 Active Learning Module\\
 +Solder-less breadboard, and jumper wire kit\\
 +2 1 kΩ resistor\\
 +1 4.7 kΩ resistors\\
 +3 10 kΩ resistors\\
 +1 10 kΩ potentiometer\\
 +1 OP97 operational amplifier\\
 +1 2N3904 npn transistor\\
 +
 +===== Voltage Controlled Amplifier using transistor =====
 +
 +==== Background ====
 +
 +Consider the circuit schematic presented in Figure 1.
 +
 +<WRAP centeralign> {{:university:courses:electronics:var_gain_amp_crl-sch.png|}} </WRAP>
 +
 +<WRAP centeralign> Figure 1. Voltage control using transistor </WRAP>
 +
 +The configuration of the circuit is similar to a basic non-inverting amplifier. The only addition consists of a transistor and a resistor in parallel with resistor R2. The transistor works as a switch that allows 2 gain settings, based on its current state (on/off).
 +
 +==== Hardware Setup ====
 +
 +Build the following breadboard circuit for the voltage-controlled amplifier using transistors.
 +
 +<WRAP centeralign> {{:university:courses:electronics:var_gain_amp_crl-bb.png|}} </WRAP>
 +
 +<WRAP centeralign> Figure 2. Voltage control using transistor breadboard circuit </WRAP>
 +
 +==== Procedure ====
 +
 +Use the first waveform generator as source Vin to provide a 2V amplitude, 1 kHz sine wave excitation to the circuit. Use the second waveform generator for controlling the transistor, providing a 2V amplitude, 1Hz square wave excitation. Supply the op amp to +/- 5V from the power supply. Configure the scope so that the input signal is displayed on channel 1 and the output signal is displayed on channel 2.
 +
 +An animated plot is presented in Figure 3.
 +
 +<WRAP centeralign> {{:university:courses:electronics:var_gain_amp_crl-wav.gif|}} </WRAP>
 +
 +<WRAP centeralign> Figure 2. Voltage control using transistor waveforms </WRAP>
 +
 +The output signal varies between two values, determined by the two gain settings, based on the state of the controlled transistor.
 +
 +===== Variable Gain Inverting Amplifier using potetiometer =====
 +
 +==== Background ====
 +
 +Consider the circuit schematic presented in Figure 4.
 +
 +<WRAP centeralign> {{:university:courses:electronics:var_gain_amp_pot2-sch.png?500|}} </WRAP>
 +
 +<WRAP centeralign> Figure 4. Variable Gain Inverting Amplifier using potetiometer </WRAP>
 +
 +On the inverting amplifier a potentiometer is used to control manually the output voltage, replacing the standard feedback resistor.
 +
 +==== Hardware Setup ====
 +
 +Build the following breadboard circuit for the voltage-controlled amplifier using transistors.
 +
 +<WRAP centeralign> {{:university:courses:electronics:var_gain_amp_pot2-bb.png|}} </WRAP>
 +
 +<WRAP centeralign> Figure 5. Variable Gain Inverting Amplifier using potetiometer - breadboard circuit </WRAP>
 +
 +==== Procedure ====
 +
 +Use the first waveform generator as source Vin to provide a 2V amplitude, 1 kHz sine wave excitation to the circuit.  Supply the op amp to +/- 5V from the power supply. Configure the scope so that the input signal is displayed on channel 1 and the output signal is displayed on channel 2.
 +
 +An animated plot is presented in Figure 6.
 +
 +<WRAP centeralign> {{:university:courses:electronics:var_gain_amp_pot2-wav.gif|}} </WRAP>
 +
 +<WRAP centeralign> Figure 6. Variable Gain Inverting Amplifier using potetiometer - waveforms </WRAP>
  
university/courses/electronics/electronics-lab-variable-gain-amplifier.txt · Last modified: 03 Jan 2021 22:21 by Robin Getz

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