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NE5230

Low voltage operational amplifier

Philips Electronics 

Philips Semiconductors

Low voltage operational amplifier

Product specification

NE/SA5230

Output Stage

Processing output voltage swings that nominally reach to less than

100mV of either supply voltage can only be achieved by a pair of

complementary common-emitter connected transistors. Normally,

such a configuration causes complex feed-forward signal paths that

develop by combining biasing and driving which can be found in

previous low supply voltage designs. The unique output stage of the

NE5230 separates the functions of driving and biasing, as shown in

the simplified schematic of Figure 3, and has the advantage of a

shorter signal path which leads to increasing the effective

bandwidth.

This output stage consists of two parts: the Darlington output

transistors and the class AB control regulator. The output transistor

Q3 connected with the Darlington transistors Q4 and Q5 can source

up to 10mA to an output load. The output of NPN Darlington

connected transistors Q1 and Q2 together are able to sink an output

current of 10mA. Accurate and efficient class AB control is

necessary to insure that none of the output transistors are ever

completely cut off. This is accomplished by the differential amplifier

(formed by Q8 and Q9) which controls the biasing of the output

transistors. The differential amplifier compares the summed voltages

across two diodes, D1 and D2, at the base of Q8 with the summed

voltages across the base-emitter diodes of the output transistors Q1

and Q3. The base-emitter voltage of Q3 is converted into a current

by Q6 and R6 and reconverted into a voltage across the

base-emitter diode of Q7 and R7. The summed voltage across the

base-emitter diodes of the output transistors Q3 and Q1 is

proportional to the logarithm of the product of the push and pull

currents IOP and ION, respectively. The combined voltages across

diodes D1 and D2 are proportional to the logarithm of the square of

the reference current IB1. When the diode characteristics and

temperatures of the pairs Q1, D1 and Q3, Q2 are equal, the relation

IOP×ION=IB1×IB1 is satisfied.

Separating the functions of biasing and driving prevents the driving

signals from becoming delayed by the biasing circuit. The output

Darlington transistors are directly accessible for in-phase driving

signals on the bases of Q5 and Q2. This is very important for simple

high-frequency compensation. The output transistors can be

high-frequency compensated by Miller capacitors CM1A and CM1B

connected from the collectors to the bases of the output Darlington

transistors.

A general-purpose op amp of this type must have enough open-loop

gain for applications when the output is driving a low resistance

load. The NE5230 accomplishes this by inserting an intermediate

common-emitter stage between the input and output stages. The

three stages provide a very large gain, but the op amp now has

three natural dominant poles — one at the output of each

common-emitter stage. Frequency compensation is implemented

with a simple scheme of nested, pole-splitting Miller integrators. The

Miller capacitors CM1A and CM1B are the first part of the nested

structure, and provide compensation for the output and intermediate

stages. A second pair of Miller integrators provide pole-splitting

compensation for the pole from the input stage and the pole

resulting from the compensated combination of poles from the

intermediate and output stages. The result is a stable,

internally-compensated op amp with a phase margin of 70 degrees.

R6

Ib1

Ib2

Ib3

Q6

Vb5

Q5

CM1B

Q4

CM1A

Vb2

Q2

Q8

Q9

R7

D1

Q7

Ib4

Ib5

D2

Figure 3. Output Stage

VCC

Q3

IOP

VOUT

ION

Q1

VEE

SL00252

1994 Aug 31

7

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