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university:courses:electronics:electronics-lab-variable-gain-amplifier [26 Mar 2018 10:39] – created Antoniu Miclausuniversity:courses:electronics:electronics-lab-variable-gain-amplifier [03 Jan 2021 22:21] (current) – fix links Robin Getz
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 ====== Activity: Variable Gain Amplifiers ====== ====== Activity: Variable Gain Amplifiers ======
  
-===== Objective: =====+===== 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. 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.
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 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. 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:vctrl_amp_transistor.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 peak-to-peak, 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_g_invert_amp_pot.png|}} </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 peak-to-peak, 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.
 +
 +By varying the value of the potentiometer, 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>
 +
 +Using this type of configuration, the output is inverted and amplified based on the feedback resistance value.
 +
 +===== Variable Gain Inverting/Non-Inverting Amplifier using potetiometer =====
 +
 +==== Background ====
 +
 +Consider the circuit schematic presented in Figure 7.
 +
 +<WRAP centeralign> {{ :university:courses:electronics:var_gain_inv_noninv.png|}} </WRAP>
 +
 +<WRAP centeralign> Figure 7. Variable Gain Inverting/Non-Inverting Amplifier using potetiometer </WRAP>
 +
 +In this amplifier configuration a potentiometer is used to control manually the output voltage, being able to invert the input by adjusting properly the potentiometer.
 +
 +==== Hardware Setup ====
 +
 +Build the following breadboard circuit for the voltage-controlled amplifier using transistors.
 +
 +<WRAP centeralign> {{:university:courses:electronics:var_gain_amp_pot-bb.png|}} </WRAP>
 +
 +<WRAP centeralign> Figure 8. Variable Gain Inverting/Non-Inverting Amplifier using potetiometer - breadboard circuit </WRAP>
 +
 +==== Procedure ====
 +
 +Use the first waveform generator as source Vin to provide a 2V amplitude peak-to-peak, 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.
 +
 +By varying the value of the potentiometer, an animated plot is presented in Figure 9.
 +
 +<WRAP centeralign> {{:university:courses:electronics:var_gain_amp_pot-wav.gif|}} </WRAP>
 +
 +<WRAP centeralign> Figure 9. Variable Gain Inverting/Non-Inverting Amplifier using potentiometer - waveforms </WRAP>
 +
 +Using this configuration, the output is amplified varying between +-Vin.
 +
 +===== Questions =====
 +
 +1. Which are the gain values for each of the circuits used in this lab activity?
 +
 +2. Based on the input signals and the computed gains, which are the expected output values? Compute and compare them with the measured values.
 +
 +3. Considering the circuit in Figure 7, how can you increase the output range above +-Vin?
 +
 +===== Further Reading =====
 +
 +<WRAP round download>
 +**Lab Resources:**
 +  * Fritzing files: [[downgit>education_tools/tree/master/m2k/fritzing/var_gain_amp_bb | var_gain_amp_bb]]
 +  * LTspice files: [[downgit>education_tools/tree/master/m2k/ltspice/var_gain_amp_ltspice | var_gain_amp_ltspice]]
 +</WRAP>
 +
 +Some additional resources:
 +  * [[adi>en/analog-dialogue/articles/x-amp-45-db-500-mhz-variable-gain-amplifier.html|X-Amp™, A New 45-dB, 500-MHz Variable-Gain Amplifier (VGA) Simplifies Adaptive Receiver Designs]]
 +  * [[adi>en/analog-dialogue/articles/reinvent-the-signal-generator.html|Two New Devices Help Reinvent the Signal Generator]]
 +
 +**Return to Lab Activity [[university:courses:electronics:labs|Table of Contents]]**
 +
 +
 +
university/courses/electronics/electronics-lab-variable-gain-amplifier.txt · Last modified: 03 Jan 2021 22:21 by Robin Getz

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