Detailed explanation of RFI rectification principle 2

Analysis Method: BJT RFI Rectification

Experiments have shown that BJT-input devices have a higher sensitivity to RFI rectification than similar devices with FET inputs, and a more analytical approach can be used to explain this phenomenon.

RF circuit designers have long known that PN junction diodes are effective rectifiers due to their nonlinear IV characteristics. Spectral analysis of the current output of a BJT transistor with an HF sine wave input shows that the closer the device is biased to the "knee," the higher the nonlinearity. This in turn makes it more effective as a detector. This is especially important in low power op amps where the input transistors are biased at very low collector currents.

The rectification analysis method for the BJT collector current is described in Reference 1 and will not be repeated here unless important conclusions are drawn. These results show that the original quadratic second-order term can be simplified to a frequency-dependent term △ic (AC) (at twice the input frequency) and a DC term △ic (DC). The latter term can be expressed using Equation 2, and the final form of the rectified DC term is:

Formula 1

This equation shows that the dc component of the second-order term is proportional to the square of the HF noise amplitude, VX, and the transistor’s quiescent collector current, IC. To illustrate this characteristic of rectification, note that a bipolar transistor operating at an IC of 1 mA and struck by a 10 mVpeak high-frequency signal will have a dc collector current change of approximately 38 μA.

Reducing the rectified collector current requires reducing either the quiescent current or the interference amplitude. Since op amp and instrumentation amplifier input stages rarely offer adjustable quiescent collector current, reducing the interference noise VX level is by far the best (and almost the only) solution. For example, reducing the interference amplitude by a factor of 2 to 5 mVpeak produces a net reduction of 4 to 1 in the rectified collector current. Clearly, this means that stray HF signals must be kept away from RFI-sensitive amplifier inputs.

Analysis Method: FET RFI Rectification

The rectification analysis method for the JFET drain current is also described in Reference 1 and will not be repeated here. A similar method is used to analyze the rectification of the FET drain current as a function of the small voltage VX applied to its gate. Equation 2 summarizes the evaluation of the second-order rectification term of the FET drain current. As with the BJT, the FET second-order term has both AC and DC components. A simplified formula for the DC term of the rectified drain current is given here, where the rectified DC drain current is proportional to the square of the magnitude of the stray signal, VX.

However, Equation 2 also shows that the difference in the degree of rectification produced by FETs and BJTs is very important.

Formula 2

However, in a BJT, the change in collector current is directly related to its quiescent collector current level, and the change in JFET drain current is proportional to the drain current IDSS at zero gate-source voltage and inversely proportional to the square of its channel pinch-off voltage VP, parameters that are geometric and process dependent. Typically, JFETs used in instrumentation amplifiers and op amp input stages are biased with a quiescent current of about 0.5 IDSS. Therefore, the change in JFET drain current is independent of its quiescent drain current and therefore independent of the operating point.

Figure 2 shows a quantitative comparison of the second-order rectification DC term between a BJT and a FET. In this example, a bipolar transistor with a unit emitter area of ​​576 μm2 is compared to a unit area JFET for 20 μA IDSS and a pinch-off voltage of 2 V. Each device is biased at 10 μA and operates at TA = 25°C.

Figure 2: Comparison of relative sensitivity between BJT and JFET

It is important to note that the change in collector current in a bipolar transistor is about 1500 times greater than the change in drain current in a JFET at the same quiescent current level. This explains why FET-input amplifiers exhibit less sensitivity to large amplitude HF excitation. Therefore, they offer more RFI rectification immunity.

Based on the above, we can conclude that since the internal circuitry of the amplifier is almost invisible to the user, it is particularly important to prevent IC circuit performance degradation due to RFI for the external circuitry of the IC.

The above analysis shows that, regardless of the type of amplifier used, RFI rectification is proportional to the square of the interference signal amplitude. Therefore, in order to minimize RFI rectification in precision amplifiers, the interference level must be reduced or eliminated before the input stage. The most direct way to reduce or eliminate interference noise is to filter appropriately.