HT9274 datasheet(5/6 Pages) HOLTEK

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HT9274

November 23, 1999

Functional Description

Input stage

The input stage of op amps are nominal PMOS

differential amplifiers (see the following dia-

gram), therefore the common mode input volt-

age can extend to VSS-0.6V. On the other hand

the common mode input voltage has to be main-

tained below (VDD-1)V to keep the input device

(M2 and M3) active. This implies that when us-

ing HT9274 as a voltage follower, the input as

well as output active range will be limited be-

tween VSS ~VDD-1V (approx.). Avoid applying

any voltage greater than VDD+0.6V or less than

VSS-0.6V to the input pins, otherwise the inter-

nal input protection devices may be damaged.

Since the input impedance of PMOS is inher-

ently very high, it can directly couple to high

impedance elements without loading effect. For

example, coupling to ceramic transducers, inte-

grating capacitor and resistor networks.

Actually the extremly high input impedance is

its major advantage over the bipolar counter-

part, in some application fields such as integra-

tors where the input current of op amp can

cause significant error, the HT9274 is a better

choice than the popular LM324.

Output stage

The HT9274 uses push-pull CMOS configura-

tion as the output stage of op amps to minimize

low power consumption and to provide ade-

quate output driving current.

Note that the output is an unbuffered struc-

ture, therefore the open loop gain will be af-

fected by the load resistor since the voltage gain

of this stage can be expressed as (gm5+

gm6)´RL.

Because of the consideration for minimized

power consumption, the max. output current is

limited to about 400mA for both source and sink

drive. This is believed to be enough for most low

power systems, however it is recommended to

use the load resistor of >10kW for normal appli-

cations. In case of heavy load driving, an exter-

nal buffer stage using bipolar transistors is

recommended.

The HT9274 is internally compensated for AC

stability and capable to withstand up to a

100pF capacitive load.

V DD

V BI A S

V SS

PM O S

N o n i n v er t i ng I n

In v e r t i n g In

T o O u t put S t age

V DD

V SS

OU T

V BI AS

Fr o m

D i ffe r e n t i a l

Am p

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