Feedforward Compensation Speeds Op Amp
A feedforward compensation method increases the slew rate of the LM101A from 0.5/ms to 10V/ms as an inverting amplifier. This extends the usefulness of the device to frequencies an order of magnitude higher than the standard compensation network. With this speed improvement, IC op amps may be used in applications that previously required discretes. The compensation is relatively simple and does not change the offset voltage or current of the amplifier. In order to achieve unconditional closed loop stability for all feedback connections, the gain of an operational amplifier is rolled off at 6 dB per octave, with the accompanying 90 degrees of phase shift, until a gain of unity is reached. The frequency compensation networks shape the open loop response to cross unity gain before the amplifier phase shift exceeds 180 degrees. Unity gain for the LM101A is designed to occur at 1 MHz. The reason for this is the lateral PNP transistors used for level shifting have poor high frequency response and exhibit excess phase shift about 1 MHz. Therefore, the stable closed loop bandwidth is limited to approximately 1 MHz

Figure 1. Standard frequency compensation Usually, the LM101A is frequency compensated by a single 30 pF capacitor between Pins 1 and 8, as shown inFigure 1. This gives a slew rate of 0.5V/ms. The feedforward is achieved by connecting a 150 pF capacitor between the inverting input, Pin 2, and one of the compensation terminals, Pin 1, as shown in Figure 2. This eliminates the lateral PNP’s from the signal path at high frequencies. Unity gain bandwidth is 10 MHz and the slew rate is 10V/ms. The diode can be added to improve slew with high speed input pulses.