0195136047.pdf

236 ANALOG BUILDING BLOCKS AND OPERATIONAL AMPLIFIERS

the device that provide the correct dc levels at the terminals. Terminals at which the dc bias is

to be connected are provided on the op-amp package with the actual biasing networks connected

internally. The manufacturer specifies the permissible range of supply-voltage values and the

corresponding op-amp characteristics.

Manufacturers add prefixes to type numbers such as 741 to indicate their own codings, even

though the specifications are similar. For example,

μA 741 Fairchild

LM 741 National Semiconductors

An extra letter is sometimes added to indicate a temperature specification. For example,

μA 741 A Military specification for guaranteed operation between−55 and+125°C

μA 741 C Commercial specification for temperature range of 0 to 70°C

Practical op amps are composed of several amplifier stages, a typical structure of which is

shown in Figure 5.3.1. The three building blocks are the input differential amplifier, the common

emitter stage, and the emitter follower output stage. Some of the op-amp practical properties

follow.

OPEN-LOOPVOLTAGEGAINA

The op amp amplifies the differencevdbetween the voltage on the noninverting (+) terminal

and the inverting (−) terminal; see Figure 5.3.2. The term “open loop” implies that there is no

external feedback connection between the output and either of the inputs.Ais defined as the

ratio of the change in output voltage to the change in differential input voltage, usually for a load

resistance of no less than 2k. A typical value forAis 2× 105.

INPUTRESISTANCERi

Input resistance is the open-loop incremental resistance looking into the two input terminals,

and is typically 2 M. Manufacturers sometimes quote the resistance between inputs and ground.

OUTPUTRESISTANCERo

The open-loop output resistance is usually between 50 and 500, with a typical value of 75

for the 741. Thus one can see that Figure 5.3.3 is the representation of an op amp as a circuit

element or block.

vp

vo

vn

Differential

amplifier

Common

emitter

stage

Emitter

follower

output

Figure 5.3.1Typical op-amp stages.

−

+

− −

+

+

+

−

vp vo^ =^ Avd^ =^ A(vp^ −^ vn)

vn

Figure 5.3.2Op-amp symbol with signal voltages.