The voltage follower, as its name suggests, is that the output voltage is the same as the input voltage, that is, the voltage follower voltage is always less than or close to one. The distinguishing feature of the voltage follower is that the input impedance is high and the output impedance is low. In general, it is easy to achieve an input impedance of several megaohms. The output impedance is low, usually up to a few ohms or even lower.
The main reason for the phase difference between the input and outputThe reasons can be roughly divided into two types:
1, due to the inherent characteristics of the operational amplifier
2, due to the characteristics of the feedback loop other than the operational amplifier
Operational amplifier characteristicsFig2a and Fig2b representatively reflect the voltage gain-frequency characteristics and phase-frequency characteristics of the operational amplifier, respectively. These two graphs are also available in the data sheet.
As shown, the voltage gain and phase of the op amp vary with frequency. The difference between the gain of the op amp and the gain after feedback (0 dB when using the voltage follower) is the gain (feedback gain) around the feedback loop. If the feedback gain is less than 1 (0 dB), then even if the phase changes by 180o, returning to the positive feedback state, the negative gain will gradually decay in the circuit, theoretically without causing oscillation.
Conversely, when the phase changes by 180o, if the loop gain corresponding to the frequency is 1x, the original amplitude will be maintained; if the loop gain corresponding to the frequency is greater than 1x, the amplitude will gradually diverge. In most cases, during amplitude divergence, the amplitude is limited by the nonlinear elements such as the maximum output voltage, and the oscillation state is maintained.
For this reason, the difference between the phase corresponding to the frequency when the loop gain is 0 dB and 180o is an important factor for judging the stability of the negative feedback loop, and this parameter is called phase margin. (Fig2b.)
Unless otherwise specified, a single amplifier must maintain a sufficient phase margin as a voltage follower.
Note: The data sheet states that the "recommended gain of more than 6dB" amplifier is not used as a voltage follower.
The effect of the peripheral circuit of the operational amplifier on the feedback loopIn practical applications, the voltage follower is not simply connected directly to the input and output as in Fig. 1. At least the output is connected to a load. Therefore, the impact of this load on the amplifier must be considered.
For example, as shown in Fig. 3, when a capacitor is connected between the output terminal and the ground, this constant causes a phase lag with the constant formed by the output resistance of the operational amplifier.
(The state shown in Fig. 2b. may change to the state shown in Fig. 2c.) At this time, the loop gain is lowered by the output resistance and C. At the same time, there is no longer a proportional relationship between phase and gain. Phase lag becomes a decisive factor, making the feedback loop unstable and, at worst, oscillating. When the capacitor is simply connected between the output and the ground to form a voltage follower, there is a difference in stability between each of the operational amplifiers.
Fig. 4. A problem that may occur with an op amp that requires a protective resistor at the input.
In order to solve the problems in Fig. 3, the methods shown in Fig. 5(a) and (b) can be used. (a) Insert R in the figure to eliminate the feedback loop phase lag caused by CL. (In the high frequency region, R appears as a load of the operational amplifier instead of CL.) (b) C1 is used to eliminate the phase lag caused by CL.
To solve the problem in Fig. 4, an appropriately sized capacitor can be connected in parallel with the input protection resistor. The approximate value commonly referred to as the "input capacitance cancellation value" is approximately 10 pF to 100 pF.
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