WEBENCH® Tools and Photodetector Stability

The first priority for photosensor applications is to achieve good stability in the transimpedance amplifier circuit. WEBENCH® Designer Tool TI developers are committed to providing customers with photosensor designs that have a 60° phase margin, which is approximately 8.7% overshoot of a step input signal.

The WEBENCH Designer tool has powerful software algorithms and a visual interface that can generate complete power, lighting, and sensor detection applications in seconds. This capability allows users to compare values ​​at the system and supply chain levels before designing. There are many tools embedded in the WEBENCH environment, one of which is the photodiode section of the "Sensor Designer". This article will specifically introduce you to the embedded photodiode circuit stability of the WEBENCH Sensor Designer.

Consequences of Ignoring Stability

Many light-sensitive applications use photodiode preamplifier (preamp) circuits. These circuits convert light information from an LED or light source into an effective voltage. When using precision photoconductance circuits with zero bias voltage (photoZB) and high-speed photoconductance circuits with negative or reverse bias voltage (photoRB), the embedded circuit phase margin is critical. Some precision photoZB applications using photodiode preamps include CT scanners, blood analyzers, smoke detectors, and position sensors. These precision circuits require voltage feedback amplifiers with low input bias current, low offset voltage, and low noise. Low-precision photoRB applications that utilize detection of digital light signals include barcode scanners and fiber optic receivers. These high-speed application circuits require voltage feedback amplifiers with greater bandwidth.

The simplest way to design a photodiode preamplifier circuit is to place the photodiode between the amplifier inputs, ground the non-inverting input, and place a resistor in the feedback loop. This allows you to configure the light-sensitive photodiode with or without a bias voltage. In the precision photoZB structure (see Figure 1a), the input amplifier needs to have a FET or CMOS input structure with low input bias current and low offset voltage. In this circuit, the photodiode cathode is connected to the amplifier inverting input, and the photodiode anode is grounded. The photodiode sensor in this circuit is zero biased. Note the direction of the current IPH with respect to the anode and cathode of the photodiode.

Figure 1 Photodiode pre-amplification structure

If digital speed and fast response time are important, the photoRB configuration (see Figure 1b) uses a reverse biased voltage on the photodiode. This reverse bias voltage creates a leakage current in the photodiode. However, the parasitic capacitance of the photodiode is considerably lower than that of the photoZB configuration. The reduction in the photodiode capacitance increases the bandwidth of the circuit. The amplifier used in the reverse biased photodiode preamplifier configuration can use FET, CMOS, or bipolar inputs; however, the higher the bandwidth of the amplifier, the better.

In either configuration, incident light on the photodiode causes a current (IPH) to flow through the diode from cathode to anode. This current also flows through the feedback resistor, RF, causing a voltage drop across the resistor. The amplifier input stage holds the amplifier inverting input at about ground level.

The simple solutions shown in Figures 1a and 1b will not usually work. Figure 2 shows how a step input light signal can produce horrible ringing at the amplifier output, VOUT. If we are lucky, this photosensitive circuit may not exhibit ringing, but it is best to understand and compensate for this stability problem.

Figure 2 Uncompensated photoZB photodiode circuit

In Figure 3, the addition of capacitor CF in the feedback loop changes the overall phase margin of the circuit and eliminates the ringing of the output signal. However, this simple solution overcompensates because the value of CF is set too high, causing the amplifier output to propagate too slowly.

Figure 3 Transition-compensated photodiode circuit

In photoZB applications, the overcompensation shown in Figure 3 may be acceptable, but it consumes more power and has higher noise than a properly compensated circuit. For photoRB applications, this circuit response is unacceptable because it does not produce a good square wave response. Since the photoRB circuit relies on a noise-free digital square wave signal, we need to pay more attention to the structures shown in Figures 2 and 3 to obtain correct compensation.

Photodiode compensation factors

The target phase margin for this transimpedance amplifier is 60°. This phase margin allows for an 8.7% overshoot in terms of step response (see Figure 4). Some designers would say that the correct phase margin for this bipolar system is 45°. As shown in Figure 4, the step response of the 45° phase margin circuit is 22.5%.

Figure 4 Relationship between overshoot response and phase margin

In theory, both phase margins allow for a stable circuit design; however, we have not yet considered variations in amplifier bandwidth, resistance, capacitance, and stray capacitance. These variations can have a significant adverse effect on a circuit with a 45° phase margin.

Correct compensation of the simple circuit shown in Figure 3 requires a clear understanding of the capacitive and resistive factors. Figure 5 shows a system model that includes a feedback network (RF and CF) and an op amp. The following discussion will show you how all the capacitive components combined have a direct impact on the frequency response of the circuit. Before installing hardware or performing manual calculations, we can first use the WEBENCH Sensor Design tool to generate a design with good system stability.

Figure 5 System model of photodetector circuit

The transfer function of the bipolar system circuit shown in Figure 5 is:

Where β is the inverse of the noise gain, that is:

To design a light sensing circuit with good stability, you need to follow some methods. WEBENCH Sensor Design Tool is powerful enough to provide you with a circuit with a stable 60° phase margin.