tlc272

Unused amplifiers should be connected as grounded unity-gain followers to avoid possible oscillation.

noise performance

The noise specifications in operational amplifier circuits are greatly dependent on the current in the first-stage
differential amplifier. The low input bias current requirements of the TLC272 and TLC277 result in a very low
noise current, which is insignificant in most applications. This feature makes the devices especially favorable
over bipolar devices when using values of circuit impedance greater than 50 k

Ω, since bipolar devices exhibit

greater noise currents.

VI

–

+

–

+

VI

(b) INVERTING AMPLIFIER

–

+

(c) UNITY-GAIN AMPLIFIER

(a) NONINVERTING AMPLIFIER

VI

OUT

OUT

OUT

Figure 40. Guard-Ring Schemes

output characteristics

The output stage of the TLC272 and TLC277 is designed to sink and source relatively high amounts of current
(see typical characteristics). If the output is subjected to a short-circuit condition, this high current capability can
cause device damage under certain conditions. Output current capability increases with supply voltage.

All operating characteristics of the TLC272 and TLC277 are measured using a 20-pF load. The devices can
drive higher capacitive loads; however, as output load capacitance increases, the resulting response pole
occurs at lower frequencies, thereby causing ringing, peaking, or even oscillation (see Figure 41). In many
cases, adding a small amount of resistance in series with the load capacitance alleviates the problem.

TLC272, TLC272A, TLC272B, TLC272Y, TLC277

LinCMOS

 PRECISION DUAL OPERATIONAL AMPLIFIERS

SLOS091E – OCTOBER 1987 – REVISED FEBRUARY 2002

31

POST OFFICE BOX 655303 

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

output characteristics (continued)

(c) CL = 150 pF, RL = NO LOAD

(b) CL = 130 pF, RL = NO LOAD

(a) CL = 20 pF, RL = NO LOAD

VI

– 2.5 V

CL

VO

2.5 V

–

+

TA = 25°C

f = 1 kHz

VIPP = 1 V

(d) TEST CIRCUIT

Figure 41. Effect of Capacitive Loads and Test Circuit

Although the TLC272 and TLC277 possess excellent high-level output voltage and current capability, methods
for boosting this capability are available, if needed. The simplest method involves the use of a pullup resistor
(RP) connected from the output to the positive supply rail (see Figure 42). There are two disadvantages to the
use of this circuit. First, the NMOS pulldown transistor N4 (see equivalent schematic) must sink a comparatively
large amount of current. In this circuit, N4 behaves like a linear resistor with an on resistance between
approximately 60 

Ω and 180 Ω, depending on how hard the operational amplifier input is driven. With very low

values of RP, a voltage offset from 0 V at the output occurs. Second, pullup resistor RP acts as a drain load to
N4 and the gain of the operational amplifier is reduced at output voltage levels where N5 is not supplying the
output current.