Transimpedance amplifier
This is the case with photodiodes where it is not uncommon for the current response to have better than 1% nonlinearity over a wide range of light input.
In its simplest form a transimpedance amplifier has just a large valued feedback resistor, Rf.
The one factor they all have in common is the requirement to convert the low-level current of a sensor to a voltage.
The gain, bandwidth, as well as current and voltage offsets change with different types of sensors, requiring different configurations of transimpedance amplifiers.
The input offset voltage due to the photodiode is very low in this self-biased photovoltaic mode.
This configuration is used with photodiodes that are illuminated with low light levels and require a lot of gain.
The DC and low-frequency gain of a transimpedance amplifier is determined by the equation so If the gain is large, any input offset voltage at the non-inverting input of the op-amp will result in an output DC offset.
An input bias current on the inverting terminal of the op-amp will similarly result in an output offset.
[3] An inverting TIA can also be used with the photodiode operating in the photoconductive mode, as shown in figure 2.
This reverse bias increases the width of the depletion region and lowers the junction capacitance, improving the high-frequency performance.
The photoconductive configuration of a transimpedance photodiode amplifier is used where higher bandwidth is required.
The frequency response of a transimpedance amplifier is inversely proportional to the gain set by the feedback resistor.
At low frequencies the feedback factor β has little effect on the amplifier response.
The peaking of the gain curve is typical of uncompensated or poorly compensated transimpedance amplifiers.
When the amplifier's 180° of phase inversion is added to this, the result is a full 360° at the fi intercept, indicated by the dashed vertical line.
Oscillation will occur at the frequency fi because of the 360° phase shift, or positive feedback, and the unity gain.
[6] To mitigate these effects, designers of transimpedance amplifiers add a small-value compensating capacitor (Cf in the figure above) in parallel with the feedback resistor.
5, where the compensated feedback factor plotted as a reciprocal, 1/β, starts to roll off before fi, reducing the slope at the intercept.
A compensation method that uses a larger-value capacitor that is not as susceptible to parasitic capacitance effects can also be used.
[9] In most practical cases, the dominant source of noise in a transimpedance amplifier is the feedback resistor.
The feedback resistance and therefore the sensitivity are thus limited by the required operating frequency of the transimpedance amplifier.
It is also possible to construct a transimpedance amplifier with discrete components using a field effect transistor for the gain element.
