The Differential Amplifier
Robert B. Northrop, Robert B. Northrop · 2003
The differential or difference amplifier (DA) is a cornerstone element in the design of most signal conditioning systems used in biomedical engineering applications, as well as in general instrumentation. All instrumentation and medical isolation amplifiers are DAs as are nearly all operational amplifiers. Why are DAs so ubiquitous? The answer lies in their inherent ability to reject unwanted dc levels, interference, and noise voltages common to both inputs. An ideal DA responds only to the so-called difference-mode signal at its two inputs. Most DAs have a single-ended output voltage, V , given by the phasor relation: V = A V - A ¢ V ¢ (3.1) Ideally, the gains A and A ¢ should be equal. In practice this does not happen; thus: V = A V + A V (3.2) where: V ∫ ( V - V ¢ )/2 (difference-mode input voltage) (3.3A) V ∫ ( V + V ¢ )/2 (common-mode input voltage) (3.3B) and A is the complex difference mode gain and A is the complex commonmode gain. It can easily be shown by vector summation that: A = A + A ¢ (3.4A) A = A - A ¢ (3.4B) Clearly, if A = A ¢ , then A Æ Figure 3.1 illustrates a simplified circuit of the Burr-Brown OPA606 JFETINPUT op amp. Note that a pair of p-channel JFETS connected as a DA are used as a differential input headstage in the op amp. The single-ended signal output from the left-hand (inverting input) JFET drives the base (input) of a BJT emitter-follower, which drives a second BJT connected as a groundedbase amplifier. Its output, in turn, drives the OA’s output stage.