5 Impact of Current Noise has on the Transimpedance Configuration

The previous section showed an example where a large source impedance is connected to the non-inverting amplifier input. A Ph sensor is a common example of a sensor that is normally connected to the non-inverting input and has impedance ranging from 10MΩ to 1000MΩ. The transimpedance amplifier is another common configuration that uses large impedance and considers current noise as an important error source. The most common application for a transimpedance amplifier is to translate the output current from a photodiode to a voltage. In many cases the photodiode current can be very low level, so a large feedback impedance is required.

In the case of the transimpedance amplifier, the amplifiers bandwidth is also severely limited with a large feedback resistor. The bandwidth of the current gain (V_out/i_in), is limited to 1.59kHz in this example ($f_c=1/\left(2\pi R_f{C}_f\right)=1.59kHz$ ). Note that Cf is a combination of the parasitic capacitance of the feedback resistor and any external capacitor added. For the transimpedance amplifier some amount of feedback capacitance is required for amplifier stability. As with the non-inverting configuration, the bandwidth limit will roll-off the noise and signal gain long before the f-squared noise begins, so the f-squared noise cannot be seen in the output spectrum.

In comparing to the noninverting case, the current noise was divided between the source impedance path and the common mode and parasitic input capacitor path. However, in the case of the transimpedance amplifier, all the current noise is forced to flow through the feedback resistor. That is, for any current developed in the parasitic capacitance on the inverting node, an equal and opposite current will flow through the feedback network. Any additional parasitic capacitance on the inverting node will cause the f-squared noise to increase for the transimpedance amplifier. This capacitance on the inverting input will not directly impact the signal bandwidth but from a practical perspective this capacitance will need to be counterbalanced by increasing the feedback capacitor for amplifier stability. Increasing the feedback capacitor on the transimpedance amplifier will reduce bandwidth. On the other hand, increasing the parasitic capacitance non-inverting input will direct reduce the bandwidth for non-inverting amplifiers. The key point is that the non-inverting amplifier will has a slight advantage over the transimpedance amplifier from a current noise perspective. Figure 5-4 illustrates a comparison between the transimpedance and non-inverting configuration for total RMS noise.