Analog designers take great pains to make amplifiers stable when they design them, but there are many situations that cause them to oscillate. Various types of loads can make them sing. Improperly designed feedback networks can cause instability. Insufficient supply bypassing can offend. Finally, inputs and outputs can oscillate by themselves as one-port systems.

Linear Technology’s LTC6268 is a low-noise 500MHz amplifier with rail-to-rail outputs and only 3fA bias current. The dominant compensation’s –90° phase lag starts at about 0.1MHz, reaches –270° at about 8MHz, but moves past –270° beyond 30MHz. In practice, all amplifiers have high-frequency phase lags additional to the basic dominant compensation lag due to extra gain stages and the output stage. Typically, the extra phase lag starts at around GBF/10.

While the LTC6268 is quite stable at unity gain, there are a few op amps that are intentionally not. By designing the amplifier compensation to be stable only at higher closed-loop gains, design trade-offs can deliver higher slew-rate, wider GBF and lower input noise than unity-gain-compensated. The LT6230-10 amplifier is intended to be used at a fed-back gain of 10 or higher, so the feedback network will attenuate the output by at least 10. With this feedback network we look for the frequency where the open-loop gain is 10V/V or 20dB and find a phase margin of 58° at 50MHz (±5V supplies). At unity gain, our phase margin is just about 0° and the amplifier will oscillate.

By observation, all amplifiers will be more stable when provided with more closed-loop gain than the minimum stable gain. Even a gain of 1.5 makes a unity-gain-stable amplifier much more stable.

In conclusion, the designer needs to consider parasitic capacitance and inductance associated with each op amp terminal and the nature of the load. The amplifiers are designed to be stable within a nominal environment, but each application requires its own analysis.

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