This article provides design tips on how to ensure amplifier stability without delving into the mathematics of the underlying principles.
Current feedback (CFB) amplifiers can exhibit high gain peaking and become unstable, and for various reasons even turn into oscillators. The two major causes for amplifier instability are making the value of the feedback resistor too low, and introducing parasitic input and output capacitances with regard to ground. While small capacitances cause the amplifier’s frequency response to peak at high frequencies, high capacitance values can force the device into self-sustaining oscillations, where it ignores any input signal stimuli.
This article provides design tips on how to ensure amplifier stability. Following the dos and don’ts enables the reader to design stable amplifier circuits without delving into the mathematics of the underlying principles.
There are three main ways to minimize the effects of parasitic capacitances on the amplifier’s stability:
Board layout tips
Achieving optimum performance with CFB amplifiers requires careful attention to board layout parasitics as well as external component types and resistor values. Referring to Figures 1 and 2, the following recommendations help optimize circuit performance:
Manufacturers of CFB amplifiers commonly specify multiple RF values, each corresponding to a different gain setting. Applying the recommended resistor values ensures optimum performance without (or only little) gain peaking or bandwidth reduction. Deviating from these values modifies amplifier performance. Figure 3 depicts this by using different RF values for a signal gain of two. The optimized and specified value of RF = 1.1kΩ for this gain shows optimal performance. However, when raising RF to 1.5kΩ, a reduction in bandwidth occurs, and when lowering RF to 600Ω, gain peaking occurs (Figure 4).
Therefore, to achieve optimum performance, follow the manufacturer’s recommended RF values.
Figure 3 Using RF values specified in the data sheet ensures best performance.
Compensating the effects of parasitic capacitance
To distinguish between the parasitic capacitance at the input (CPI) and the output (CPO), the pulse-response test can be applied. CPI, which is usually smaller than CPO, causes short signal overshoots, while CPO often results in prolonged signal ringing (Figure 5). Of course, the situation is reversed if CPI > CPO. This however, is rarely the case.
[Continue reading on EDN US: Parasitic input capacitance]
Tom Kugelstadt is a principal applications engineer with Renesas Electronics America where he defines new high-performance analog products for industrial systems.