Operational amplifiers (op amps) are well enough known in the art as high gain devices for providing an output signal in response to a differential input signal. It is desirable for the op amp to have a high gain-bandwidth product for providing satisfactory operation over a wide band of frequencies. Since the gain-bandwidth product of the op amp remains substantially constant at any given point of operation, a higher feedback loop gain generally reduces the operational bandwidth while a higher operational bandwidth requires a smaller feedback loop gain to avoid oscillations, as is well understood. Although the feature of high gain alone is not difficult to achieve, the op amp must also maintain stable operation over the specified bandwidth. Thus, there is a tradeoff between offering a wide operational bandwidth and having high gain for the op amp with stability over the entire frequency range being of paramount concern.
Two generally accepted criteria for judging the degree of stability of the op amp are the phase margin and gain margin which may be read from a Bode plot diagram of the magnitude of the gain and the phase versus radian frequency .omega.. The phase margin is the difference between 360.degree. (degrees) and the phase response of the transfer function at the radian frequency corresponding to 0 dB (decibels) of the gain response, and the gain margin is the difference between 0 dB and the gain across the op amp at the critical frequency corresponding to 360.degree. of the phase response. For all frequencies at which the gain is greater than one (0 dB), it is important to have less than 360.degree. of phase shift through the amplifier; otherwise, when the output of the op amp is coupled back to its input by some impedance network, for example in an active filter application, the output signal could feedback in-phase with the input signal thereby permitting the op amp to sustain its own operation and oscillate.
The transfer function of an uncompensated op amp plotted as the gain versus radian frequency .omega. naturally rolls off at some high frequency as the components thereof reach an operational limit. However, such uncompensated op amps typically possess unacceptable gain and phase margins as one or more frequencies of operation pass through 360.degree. of phase shift with an open loop gain greater than 0 dB, leaving the op amp exposed to possible oscillations. Consequently, most op amps are compensated with a dominate pole inserted at a predetermined low frequency to ensure that the transfer function rolls off at say 6 dB per octave in frequency and drops below 0 dB before the phase response reaches 360.degree. . Earlier versions of op amps provided external ports at which a capacitor could be coupled for providing the dominate pole. More recently, op amps have been internally compensated, for example, with a capacitor feedback circuit matched to the particular characteristics of the op amp for maximizing the gain-bandwidth product. The value selection of the compensating capacitor and the predetermined low frequency of the dominate pole is important in determining the performance of the op amp. The frequency of the dominate pole should be made as high as possible for providing a good bandwidth, yet a higher frequency selection extends the roll-off frequency (first break point of the transfer function) and the unity gain frequency (0 dB crossing) thereby lowering the gain and phase margin. If the frequency of the dominate pole is selected too high such that component variation in the manufacturing process could allow the frequency of the dominate pole to drift even higher, the phase response may pass through 360.degree. before the gain response falls below 0 dB and the op amp may operate in an unstable condition.
Occasionally, a double pole is inserted at the predetermined low frequency for providing an even steeper roll-off of the gain response, say 12 dB/octave, which might seem to offer a higher roll-off frequency without increasing the unity gain frequency and providing a wider bandwidth. However, the double poles also inserts a 180.degree. shift causing the phase response to approach the critical 360.degree. at a lower frequency as compared to a single pole response, possibly creating an unacceptable phase margin and again leaving the op amp susceptible to oscillation thereby defeating the purpose of the double pole. Thus, most op amp designers opt for the first alternative of only a single dominate pole for the internal frequency compensation circuit with sufficient guard band in the gain and phase margins to allow for component variation in the manufacturing process and accept the associated limitation on the gain-bandwidth product.
Hence, what is needed is an improved frequency compensation circuit for op amps which allows the dominate pole to be located at a higher frequency for increasing the gain-bandwidth product while ensuring the stability of the op amp over the operational bandwidth.