ADALM2000 Activity: Variable Gain Amplifiers

Objective

In this laboratory, we continue our discussion on operational amplifiers (see the previous lab “ADALM2000 Simple Op Amps”), focusing on variable gain/voltage controlled amplifiers.

Most operational amplifier (op amp) circuits have a fixed level of gain. However, it is often advantageous 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 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 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
  • Solderless breadboard and jumper wire kit
  • Two 1 kΩ resistors
  • One 4.7 kΩ resistor
  • Three 10 kΩ resistors
  • One 10 kΩ potentiometer
  • One OP97 operational amplifier
  • One 2N3904 NPN transistor

Voltage Controlled Amplifier Using a Transistor

Background

Consider the circuit schematic presented in Figure 1.

Figure 1. Voltage control using a transistor.

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 two gain settings, based on its current state (on/off).

Hardware Setup

Build the following breadboard circuit for the voltage controlled amplifier using transistors (Figure 2).

Figure 2. Breadboard circuit of voltage control using a transistor.

Procedure

Use the first waveform generator as source VIN to provide a 2 V amplitude peak-to-peak, 1 kHz sine wave excitation to the circuit. Use the second waveform generator for controlling the transistor, providing a 2 V amplitude, 1 Hz square wave excitation. Supply the op amp to ±5 V 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. See Figure 3.

Figure 3. Waveforms of voltage control using a transistor.

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 a Potentiometer

Background

Consider the circuit schematic presented in Figure 4.

Figure 4. A variable gain inverting amplifier using a potentiometer.

On the inverting amplifier, a potentiometer is used to manually control the output voltage, replacing the standard feedback resistor.

Hardware Setup

Build the following breadboard circuit for the voltage controlled amplifier using transistors (Figure 5).

Figure 5. Breadboard circuit of a variable gain inverting amplifier using a potentiometer.

Procedure

Use the first waveform generator as source VIN to provide a 2 V amplitude peak-to-peak, 1 kHz sine wave excitation to the circuit. Supply the op amp to ±5 V 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. See Figure 6.

Figure 6. Waveforms of a variable gain inverting amplifier using a potentiometer.

Using this type of configuration, the output is inverted and amplified based on the feedback resistance value.

Variable Gain Inverting/Non-Inverting Amplifier Using a Potentiometer

Background

Consider the circuit schematic presented in Figure 7.

Figure 7. Variable gain inverting/non-inverting amplifier using a potentiometer.

In this amplifier configuration, a potentiometer is used to manually control the output voltage, inverting the input by properly adjusting the potentiometer.

Hardware Setup

Build the following breadboard circuit for the voltage controlled amplifier using transistors (Figure 8).

Figure 8. Breadboard circuit of a variable gain inverting/non-inverting amplifier using a potentiometer.

Procedure

Use the first waveform generator as source VIN to provide a 2 V amplitude peak-to-peak, 1 kHz sine wave excitation to the circuit. Supply the op amp to ±5 V 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. See Figure 9.

Figure 9. Waveforms of a variable gain inverting/non-inverting amplifier using a potentiometer.

Using this configuration, the output is amplified varying between ±VIN.

Question

Can you briefly name several real-life applications in which variable gain amplifiers are useful?

You can find the answer at the StudentZone blog.

作者

Antoniu Miclaus

Antoniu Miclaus

Antoniu Miclaus现为ADI公司的系统应用工程师,从事ADI教学项目工作,同时为Circuits from the Lab®、QA自动化和流程管理开发嵌入式软件。他于2017年2月在罗马尼亚克卢日-纳波卡加盟ADI公司。他目前是贝碧思鲍耶大学软件工程硕士项目的理学硕士生,拥有克卢日-纳波卡科技大学电子与电信工程学士学位。