HT9231/HT9232/HT9234

Rev. 1.10

4

April 8, 2010

Functional Description

Input stage

As the input stage of these op-amps is a PMOS differen-

tial amplifiers, the amplifiers can offer an extended com-

other end of the voltage spectrum, the common mode

input voltage has to be maintained at a level below

-1.4 to keep the input devices, M2 and M3, in their

active region. This implies that when using

HT9231/HT9232/HT9234 in voltage follower applica-

tions, the inputs as well as the output active range will be

nal input protection devices may be damaged.

Since the input impedance of a PMOS transistor is in-

herently very high, they can be directly coupled to high

impedance elements without loading effects. Examples

could be coupling to ceramic transducers, integrating

capacitors and resistor networks. It is this high input im-

pedance characteristic that is its major advantage over

its bipolar counterpart in certain application fields such

as integrators where op-amp input currents can cause

significant errors.

Output stage

The devices use a push-pull CMOS configuration for the

op-amp output stage to minimise power consumption

and to provide adequate output drive currents. Note that

the output is an unbuffered structure, therefore the open

loop gain will be affected by the load resistor since the

voltage gain of this stage is expressed as (gm5+

gm6)

To keep power consumption to a minimum, the output

short circuit current is limited to about -9mA for the

source drive and 9.5mA for the sink drive. This is consid-

ered to be enough for most low power systems, how-

ever it is recommended only to used load resistors of

>10k

W for most applications. For heavy load driving ap-

plications, an external buffer stage using bipolar transis-

tors is recommended.

The HT9231/HT9232/HT9234 is internally compen-

sated for AC stability and can drive capacitive loads of

up to 60pF.

M 1

M 2

M 3

M 4

M 5

P M O S

P M O S

N o n i n v e r t i n g I n

I n v e r t i n g I n

T o O u t p u t S t a g e

M 5

M 6

O U T

F r o m

D i f f e r e n t i a l

A m p