HT9291/HT9292/HT9294

Rev. 1.00

4

March 8, 2010

Functional Description

Input Stage

The input stage of op amps are nominal PMOS differen-

tial amplifiers (see the following diagram), therefore the

On the other hand the common mode input voltage has

-1.2V) to keep the input de-

vice (M2 and M3) active. This implies that when using

HT9291/HT9292/HT9294 as a voltage follower, the in-

put as well as output active range will be limited be-

the input pins, otherwise the internal input protection de-

vices may be damaged.

Since the input impedance of PMOS is inherently very

high, it can directly couple to high impedance elements

without loading effect. For example, coupling to ceramic

transducers, integrating capacitor and resistor net-

works.

Actually the extremly high input impedance is its major

advantage over the bipolar counterpart, in some appli-

cation fields such as integrators where the input current

of op amp can cause significant error.

Output Stage

The HT9291/HT9292/HT9294 uses push-pull CMOS

configuration as the output stage of op amps to mini-

mize low power consumption and to provide adequate

output driving current.

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 can be expressed as

(gm5+ gm6)

Because of the consideration for minimized power con-

sumption, the output short circuit current is limited to

about -1.2mA for source drive and 4mA for sink drive.

This is believed to be enough for most low power sys-

tems, however it is recommended to use the load resis-

tor of >1M

W for normal applications. In case of heavy

load driving, an external buffer stage using bipolar tran-

sistors is recommended.

The HT9291/HT9292/HT9294 is internally compen-

sated for AC stability and capable to withstand up to a

60pF capacitive load.

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