Which statement correctly distinguishes transimpedance amplifiers from transconductance amplifiers?

Which statement correctly distinguishes transimpedance amplifiers from transconductance amplifiers?

• A. Transimpedance converts current to voltage: Vout = -Iin × Rf; Transconductance converts voltage to current: Iout = gm Vin; Applications include photodiode-to-voltage conversion versus V-to-I DAC or current-source stages.
• B. Transimpedance converts voltage to current; Transconductance converts current to voltage; Applications include audio amplification and audio filtering.
• C. Both amplifiers convert voltage to current, but transimpedance is faster.
• D. They are identical in function; the terms are interchangeable.

Answer: (A)

The main idea here is how each amplifier type maps input signals to output signals. A transimpedance amplifier converts input current into an output voltage. In the common inverting op-amp setup, the output voltage is proportional to the input current, Vout ≈ -Iin × Rf, where Rf is the feedback resistor. This makes it ideal for sensing current from devices like photodiodes, turning a tiny photocurrent into a usable voltage while keeping the input at a virtual ground and preserving current information. A transconductance amplifier does the opposite: it converts an input voltage into an output current, with Iout = gm × Vin. Here gm is the transconductance, so the device acts as a voltage-controlled current source. This arrangement is useful in scenarios like DAC output stages or current-source generation where you need a controllable current based on a voltage input. Because they operate in opposite directions (current-to-voltage vs voltage-to-current) and have different useful applications, they are not interchangeable. The transimpedance gain has units of ohms (V/A), reflecting a voltage output per unit input current, while the transconductance gain has units of siemens (A/V), reflecting current per unit input voltage.

The main idea here is how each amplifier type maps input signals to output signals. A transimpedance amplifier converts input current into an output voltage. In the common inverting op-amp setup, the output voltage is proportional to the input current, Vout ≈ -Iin × Rf, where Rf is the feedback resistor. This makes it ideal for sensing current from devices like photodiodes, turning a tiny photocurrent into a usable voltage while keeping the input at a virtual ground and preserving current information.

A transconductance amplifier does the opposite: it converts an input voltage into an output current, with Iout = gm × Vin. Here gm is the transconductance, so the device acts as a voltage-controlled current source. This arrangement is useful in scenarios like DAC output stages or current-source generation where you need a controllable current based on a voltage input.

Because they operate in opposite directions (current-to-voltage vs voltage-to-current) and have different useful applications, they are not interchangeable. The transimpedance gain has units of ohms (V/A), reflecting a voltage output per unit input current, while the transconductance gain has units of siemens (A/V), reflecting current per unit input voltage.