1. Field of the Invention
The present invention relates to amplifiers in general and in particular to techniques and devices for controlling the operating point of said amplifiers without adversely affecting the amplifiers gain and/or bandwidth.
2. Prior Art
The quiescent output voltage of a differential amplifier is typically a function of a number of device parameters, such as tolerances, temperature and supply voltage. This variation is increased if one or more emitter or source followers are added to the output to increase drive capability of the amplifier.
Traditional techniques for improving the DC accuracy include the use of overall negative feedback. There are, however, undesirable side effects resulting from the use of overall feedback which cannot be tolerated in some applications. For example, applying negative feedback to an amplifier reduces its useful signal gain which may increase the need for additional gain stages if a desired gain is to be maintained. Negative feedback around multiple stages can then cause the need to "throw away" or discard high frequency gain by using various frequency compensation techniques. For this reason high frequency (video and RF) amplifiers often use localized feedback loops. If the high frequency amplifier has multiple stages, it usually requires multiple individual feedback loops. With multiple individual feedback loops the DC errors of each loop can accumulate into relatively large errors.
FIG. 1 shows another prior art technique which is used to improve an amplifier DC accuracy without compromising and/or sacrificing bandwidth. An input signal labeled V.sub.IN is applied to the negative input terminal of amplifier 10. The gain of amplifier 10 is set by resistors RF and Rin. The average DC value at the output of amplifier 10 is sampled by an integrator, formed from amplifier 12, R.sub.int and C.sub.int, and applied to the non-inverting terminal of amplifier 10.
Even though the circuitry of FIG. 1 works well for its intended purpose, it has several drawbacks. First, the integrating feedback technique actually decreases the amplifiers DC gain. Second, the integrator must generally have a relatively long time constant. For a continuous time integrator an integrating capacitor within the range of tens of microfarads typically is required. This size capacitor would require a relatively large surface area and as such would be too large to be economically reproduced on an integrated circuit chip.
U.S. Pat. No. 4,621,238 uses a filtering circuit in a feedback loop for controlling the operating point of an amplifier. In particular, the differential output operating point of the amplifier is controlled by filtering the differential output and adjusting the current that is shared by a pair of differential devices.