Audio Engineering

Valve (Tube-Based) Amplifi ers 353

either open-circuit (O/C) or short-circuit (S/C), but with some internal resistance, shown
asR^3 [or R 1 in the case of Figure 11.8(b) ]. I have also adopted the convention that the
current fl ow into the load resistor ( R 2 ) is deemed to be positive when the amplifi er circuit
is feeding current (as a current source) into the load and to be negative when the amplifi er
is acting as a current sink and drawing current from R 2 and its associated power supply.
I have also labeled the voltage at the junction of these three resistors as Vx. The effi ciency
of the system can be considered as related to the extent of the change in the current
throughR 2 brought about by the change from O/C to S/C in Q 1 or Q 2.

If we consider fi rst the single-ended layout of Figure 11.8(a) , when Q 1 is O/C, the current
fl ow into R 2 is only through R 1 and i 2  V /( R 1  R 2 ). If, however, Q 1 is short circuited,
S/C, then, from inspection,

iii 312  (11.1)

buti x

VV

(^2) R 2





(11.2)

Figure 11.8 : Output arrangements.

0V

R 1

R 2

I 3 R 3

I 2

I 1

Q 1

V

2V

(a) Single-ended

0V

R 2

R 1

I 3 R 3

I 1

Q 1

Vx

I 2

Q 2

2V

V

(b) Push-pull