“An amplifier is the component of choice when a signal needs gain. For voltage feedback and fully differential amplifiers, the ratio of the feedback and gain resistors, RF/RG, determines the gain. After a certain ratio is set, the next step is to choose the value of RF or RG. The choice of RF can affect the stability of the amplifier.
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An amplifier is the component of choice when a signal needs gain. For voltage feedback and fully differential amplifiers, the ratio of the feedback and gain resistors, RF/RG, determines the gain. After a certain ratio is set, the next step is to choose the value of RF or RG. The choice of RF can affect the stability of the amplifier.
The amplifier’s internal input capacitance, which can be found in the data sheet spec sheet, interacts with RF to form a pole in the transfer function. If RF is extremely large, this pole will affect stability. If the pole occurs at a frequency much higher than the crossover frequency, stability will not be affected. However, if the pole position determined by f = 1/(2πRFCin,amp) occurs near the crossover frequency, the phase margin will be reduced, possibly causing instability.
The example in Figure 1 shows laboratory results of small-signal closed-loop gain and frequency response of the ADA4807-1 voltage-feedback amplifier in a noninverting gain of 2 configuration with feedback resistors of 499 Ω, 1 kΩ, and 10 kΩ. The data sheet recommends an RF value of 499.
Figure 1. Laboratory results using different feedback resistors.
VS = ±5 V, VOUT = 40 mV pp, RLOAD = 1 kΩ for RF values of 499 Ω, 1 kΩ, and 10 kΩ
The degree of peaking in the small-signal frequency response indicates instability. Increasing RF from 499 ? to 1 k? increases peaking slightly. This means that an amplifier with an RF of 1 kΩ has sufficient phase margin and is relatively stable. This is different when the RF is 10 kΩ. A high level of peaking implies instability (oscillation) and is therefore not recommended.
Figure 2. Simulation results using the ADA4807 SPICE model.
VS = ±5 V, G = 2, RLOAD = 1 kΩ for RF values of 499 Ω, 1 kΩ, and 10 kΩ.
Validating the circuit in the lab is not a mandatory step to test for potential instability. Figure 3 shows the simulation results using the SPICE model, with the same RF values of 499 Ω, 1 kΩ, and 10 kΩ. The results are consistent with Figure 1. Figure 3 shows the instability in the time domain.
Figure 3. Impulse response simulation results using the ADA4807 SPICE model.
VS = ±5 V, G = 2, RLOAD = 1 kΩ for RF values of 499 Ω, 1 kΩ and 10 kΩ
The instability shown in Figure 4 can be removed by adding a zero to the transfer function by placing a feedback capacitor across RF.
Figure 4. Impulse response simulation results using 3.3 pF feedback capacitor CF.
VS = ±5 V, G = 2, RF = 10 kΩ, RLOAD = 1 kΩ.
There are tradeoffs in the choice of RF, namely power consumption, bandwidth and stability. If power dissipation is important and the data sheet recommends that the feedback value cannot be used, or a higher RF value is required, an optional feedback capacitor can be placed in parallel with RF. This selection yields lower bandwidth.
System requirements need to be considered when selecting RF for voltage feedback and fully differential amplifiers. If speed is not important, the feedback capacitor can help stabilize larger RF values. If speed is important, it is recommended to use the RF values recommended in the data sheet. Neglecting RF’s relationship to stability, bandwidth, and power can hinder a system, or even prevent it from achieving full performance.
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