If the differential output is not desired, then only one output can be used (taken from just one of the collectors (or anodes or drains), disregarding the other output; this configuration is referred to as single-ended output. The gain stage's pulldown should be an NFET matched to the diff amp's NFETs. In common mode, the emitter voltage follows the input voltage variations; there is a full negative feedback and the gain is minimum. While if the output is taken between one collector with respect to ground it is called unbalanced output or single ended output. A differential amplifier is an op amp circuit which is designed to amplify the difference input available and reject the common-mode voltage. Compare to the folded cascode, which "folds" MP1 & MP2 downward into a pair of NMOS pulldowns. A difference amplifier is a special purpose amplifier designed to measure differential signals, otherwise known as a subtractor. 2nd stage shown above fixes this limitation - 2nd stage output is full-swing. It is used for suppressing the effect of noise at the output. PBIAS is reference gate voltage that mirrors a current from a reference cicruit that must be designed separately. V Increased mismatch sensitivity (2 mirrors in series). This does not seem to be as common in the industry as the non-cascode version, except in cases when the cascodes serve as voltage protection. In Audio related electronics pre-amplifier and power amplifiers are two different types of amplifier systems which are used for sound amplification related purposes. Shown: Differential to Single-Ended with PMOS Inputs. in 4 shows the transmission characteristic of this circuit. This circuit is commonly referred to as a diff amp or as a long-tailed pair. * An ideal differential amplifier has zero common-mode gain (i.e., A cm =0)! Assume VCC=2.5V. Higher than folded cascode, since there is no divsion by 2 of PMOS, Output Swing: 2VDsatN < Vout < VDD - 2VDsatP, Vcm < VDD - 2VDsatP - VTp to avoid cutoff of input pair, Vcm > 2VDsatN - VTp to avoid triode condition in NMOS pulldowns. But the practical value of single-ended amplifier configurations is a different story—the fact is, differential amplifiers dominate modern analog ICs. This occurs when VGS - VTn - VDsatN < VDS for an NMOS, or VSG - VTp - VDsatP, A decrease down to zero would kill the amp. The gain is half that of the stage with differential output. It also implies that the common-mode input bias current has cancelled out, leaving only the input offset current IΔb = 'I+b - 'I−b still present, and with a coefficient of Ri. CH 10 Differential Amplifiers 18 Example 10.5 A bipolar differential pair employs a tail current of 0.5 mA and a collector resistance of 1 kΩ. there is only one mirror. If the resistor at the collector is relatively large, the transistor will saturate. Thus, the difference is twice the individual signal currents (ΔI - (-ΔI) = 2ΔI) and the differential to single ended conversion is completed without gain losses. When the input is zero or negative, the output is close to zero (but can be not saturated); when the input is positive, the output is most-positive, dynamic operation being the same as the amplifier use described above. This chapter discusses the characteristics and types of op-amps. V An Op-amp (operational amplifier) is the device that possesses linear characteristics. The output impedance of the differential pair is high (especially for the improved differential pair with a current mirror as shown in Figure 3). Like CE amplifier the differential amplifier is a small signal amplifier. The name "differential amplifier" should not be confused with the "differentiator", also shown on this page. The gain stage's pulldown should be a current source matched to the diff amp's current source, such that the current density through the PFET matches that of the PFETs of the diff amp. This transconductance gm(amp) = N * gm(MP1) = N * gm(MP2). Because of this gain stage, Vout can swing as high as VDD - VDsatP and as low as VDsatN. e The Differential Amplifier, as the name suggests, amplifies the difference in the two input signals vin1 and vin2. out Deviations from these rules will cause an input offset. Differential Amplifiers: Differential amplifier is a basic building block of an op-amp. An operational amplifier, or op-amp, is a differential amplifier with very high differential-mode gain, very high input impedance, and low output impedance. 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