Transconductance amplifier is a voltage-to-current converter, that is, input voltage is converted to output current. It is a type of negative feedback amplifier and is also called Voltage Controlled Current Source(VCIS) amplifier. Other types of negative feedback amplifier are the voltage amplifier(VCVS), transimpedance amplifiers(ICVS) and current amplifier(ICIS).
The circuit of transconductance amplifier implemented with LM358 op-amp is shown below.
Mathematics
The relation between the input voltage\(v_{in}\) and the output current\(i_{out}\) is,
\(i_{out} = \frac{v_{in}}{R_1+(R_1+R2)/A_{OL}}\)
In practice the second second term of the denominator is much smaller than the first and so we can approximate the output current as,
\(i_{out} = \frac{v_{in}}{R_1}\)which is often rewritten as,
\(i_{out} = g_m v_{in}\)
where, \(g_m = \frac{1}{R_1}\)
For example, suppose the input voltage is 2Vrms then we can calculate the output current as follows,
\(i_{out} = \frac{v_{in}}{R_1}= \frac{2V_{rms}}{1\Omega}=2A_{rms}\)
The peak to peak voltage can be calculated as follows,
\(V_{pp} = 2\sqrt{2}V_{rms} \approx 5.66V\)
The following shows the waveform of the input voltage and the output current when the input voltage applied 2Vrms with 1khz frequency.
Applications of Transconductance Amplifier
A transconductance amplifier, which converts an input voltage signal into an output current, finds wide application in analog signal processing and instrumentation. It is commonly used in operational transconductance amplifiers (OTAs) for voltage-controlled amplification, where the gain can be adjusted using a control current or voltage, making it ideal for analog multipliers, modulators, and filters in communication systems. In sensor interfaces, such amplifiers are used to convert small voltage changes from transducers into proportional currents for further processing. They are also critical in active filter design, analog computation circuits, and function generators due to their linear current-output behavior.
