Detailed Analysis of the Design of Integrated Op Amp Phase Compensation Circuit

Op amp phase (frequency) compensation circuit design

The inside of the integrated op amp is a multistage amplifier. Its logarithmic amplitude-frequency characteristic is shown as curve 1 (solid line) in Fig. 1. The turning point of the logarithmic amplitude-frequency characteristic curve above zero decibel is called the pole. In the figure, the P1 P2 point is called the pole. The frequency corresponding to the pole is called the corner frequency, such as fp1, fp2, and the first pole, that is, the pole with the lowest frequency is called the main pole. At the pole, the output signal is 45° behind the input signal, and the amplitude-frequency characteristic curve varies by -20dB/10 octave slope. The output signal per decade is delayed by 90 from the input signal. The more poles, the easier it is to be self-excited, the more unstable it is. In order to stabilize the integrated op amp, phase (frequency) compensation is required.

According to the compensation principle, it is divided into lag compensation, lead compensation and lag-advance compensation.

Hysteresis compensation: Any compensation that increases the phase shift is called hysteresis compensation. Hysteresis compensation reduces the main pole frequency, ie the amplifier band is narrowed. If there is only one pole after compensation, it is called a single pole, as shown in curve 2. in Figure 2.21(a). Leading compensation: Any compensation that reduces the phase shift is called advance compensation. The lead compensation causes the amplitude-frequency characteristic curve to appear zero, that is, the amplifier frequency band becomes wider. The output signal at the zero point is 45° ahead of the input signal, and the amplitude-frequency characteristic curve changes by +20dB/10 octave slope. The compensation method is to coincide the zero point with a pole before the compensation, as shown in Fig. 2.21(a), the P2 point. The compensated amplitude-frequency characteristic curve is shown in the curve 3 in Figure 2.21(a).

1. Hysteresis compensation network at the input end (external hysteresis compensation) The network of resistors (RB) and capacitors (CB) connected in series at the input terminals of the integrated op amp is called the lag compensation at the input end. This compensation narrows the passband and is suitable for circuits that do not require high frequency bands. This approach also helps to increase the rate of increase of the integrated op amp. RB, CB estimation method (I) temporarily short CB under the condition given by the amplifier gain, parallel RB between the two input terminals of the integrated operational amplifier, the value of RB changes from large to small until the amplifier enters a critical stable state. . An approximate sine wave can be seen with the oscilloscope at this time. The oscillation period is measured by the horizontal (time) axis of the oscilloscope, and the oscillation frequency fo is actually the frequency at which the amplification factor of the amplifier is equal to one. The value of the compensation capacitor CB can be estimated by the following formula, ie

CB"1/(RB*f)

2. Feedback end lead compensation

The compensation capacitor is applied to the external feedback resistor of the closed-loop amplifier. The compensation principle is shown in curve 3 of Figure 2.21(a). This compensation is used to broaden the high-frequency bandwidth. The circuit diagram is shown in Figure 2.2.13.

(1) offset the compensation of the second pole

(2) The compensation that weakens the influence of the input distributed capacitance will compensate the capacitance and be on the external feedback resistor of the closed-loop amplifier, so that the input signal can be directly coupled to the output at high frequency, weakening the influence of the input distributed capacitance and improving the high-frequency characteristics of the circuit. The circuit diagram is shown in Figure 2.2.14. The compensation condition is RF*CB = Rr*Cr

Where r is the distributed capacitance at the input.