How to choose analog-signal-chain components: part 5

Transimpedance and transconductance op amps convert currents to voltages and voltages to currents.

Figure 1. The transfer function of this transimpedance amplifier equals -RF.

We concluded part 4 of this series with a look at the instrumentation amplifier. Like the other op-amps and filter circuits we have so far described, the instrumentation amp is a voltage-input, voltage-output device. Sometimes, however, we may need a current-input voltage-output device, called a transimpedance amplifier [1], or a voltage-input current-output device, called a transconductance amplifier[2] or voltage-controlled current source.

Q: How does a transconductance amplifier work?
A: Figure 1 shows the basic configuration. Because we know that V- » V+ » 0, we can derive the transfer function in volts per ampere as follows:

Q: What’s a typical application for a transimpedance amplifier?
A: Some sensors are best considered as current-output devices — that is, their output current varies linearly with the parameter being measured. For example, a photodiode-based photodetector generates a short-circuit current that’s proportional to incident photons. If we substitute the photodetector for the current source in Figure 1, the output voltage then represents the level of illumination.

Figure 2. A voltage drop across the photodiode compromises linearity.

Q: Why not just drive a resistor directly with the photodiode?
A: We could do that, as shown in Figure 2, if the photodiode can generate the maximum output voltage you want without going into saturation. Even if you avoid saturation, though, you would be compromising linearity, because linearity decreases as output current increases, which increases the voltage across the photodiode. As you see in Figure 1, the voltage across the photodiode remains near zero regardless of the diode’s output current.

Q: That makes sense for a photodetector, but obviously, this zero-voltage operating point won’t work for a solar cell.
A: Right. Solar cells and photodiodes both work on the principle of generating current in response to incident photons. But for a solar cell, we don’t care about linearity — we just want to extract the most power for a given level of illumination. In Figure 3, the blue curve shows a typical current-voltage (I-V) curve for a solar cell. As you note, at the short-circuit-current point (I_SC), the current is at its maximum, but voltage and therefore power are zero. At the open-circuit-voltage point (V_OC), the voltage is maximum, but the current and therefore the power are zero. The maximum power point (MPP) lies somewhere in between.

Figure 3. For a solar cell with the I-V curve shown in blue, the MPP occurs at the vertical red arrow.

In the figure, the maximum power is approximately 70% of the open-circuit voltage (V_OC) times the short-circuit current (I_SC), and the MPP occurs when the per-unit voltage and current are 0.78 and 0.89, respectively.

Q: What’s an application for the transconductance amplifier?
A: One that comes to mind, since we’re on the topic of solar cells, is photovoltaic (PV) cell simulation. If you have full characterization data for a solar cell under various conditions of illumination, you can use the transconductance amplifier to play back various conditions of illumination in the lab without needing the sun and the clouds to cooperate. This ability can be useful in the design and test of downstream power conversion or inversion circuitry (Figure 4).

Figure 4. A transconductance amplifier can facilitate a PV inverter test.

With a simple adjustable analog input, a transconductance amplifier can simulate a PV cell under all possible conditions of illumination. The simulation capability can be particularly useful for developing a power-conversion device’s MPP tracking (MPPT) algorithm, which ensures that the device remains operating at the MPP under various lighting conditions. For high-power PV cells and panel strings, however, you would want to substitute a switch-mode programmable power supply in a constant-current output mode for the linear amplifier to boost efficiency. Even a residential PV microinverter can have a rating of several hundred watts.

Q: What are some other applications?
A: Transconductance amplifiers can be used to build filters and oscillators.^[3] Both transconductance and transimpedance amplifiers are also useful in process-control systems that use 4-20 mA current loops to carry commands and sensor outputs. Each loop can carry only one variable, but the technique remains useful because of its noise immunity and reliability. Figure 5 shows one current loop carrying commands to a pump, while another carries data from a flow-rate sensor back to the controller. With these loops, 20 mA equals 100% full scale and 4 mA equals 0% full scale, eliminating the ambiguity of not knowing whether a 0-mA reading would indicate 0% full scale or an open or short circuit.

Figure 5. 4-20 mA current loops carry commands (blue) and sensor data (red).

In such a system, a transconductance amplifier can convert the voltage output of a sensor such as a thermocouple to the 4-20 mA level, and a transimpedance amplifier can convert the current signal to a voltage level required by a controller or meter. Rather than employ the configuration in Figure 1, a current-loop transimpedance amplifier makes use of a current-sense resistor, as we discussed in part 2, to avoid the problem of voltage translation. In 4- to 20-mA systems, the transconductance and transimpedance amplifiers are generally called transmitters and receivers, respectively.