Block-diagram solutions for vacuum-tube circuits

MANY AUTHORS1¿7 of textbooks on electronics begin their discussion of feedback amplifiers with the block diagram given by Black.8 The general gain equation $A^{prime} = {A over 1-AB} eqno{hbox{(1)}}$ in which A is the gain without feedback and ß is the feedback fraction, is then derived, and the beneficial effects of feed back are discussed. In dealing with specific circuits, however, it is a common practice to write and solve the circuit equations, and then to manipulate the resulting gain equation into a form in which A and ß can be identified. In some cases, expressions for A and ß are simply stated and verified by substitution into the general equation.