APEX MICROTECHNOLOGY CORPORATION • 5980 NORTH SHANNON ROAD • TUCSON, ARIZONA 85741 • USA • APPLICATIONS HOTLINE: 1 (800) 546-2739

4

OPERATING

CONSIDERATIONS

PB51 • PB51A

STABILITY

Stability can be maximized by observing the following

guidelines:

1. Operate the booster in the lowest practical gain.

2. Operate the driver amplifier in the highest practical effective

gain.

3. Keep gain-bandwidth product of the driver lower than the

closed loop bandwidth of the booster.

4. Minimize phase shift within the loop.

A good compromise for (1) and (2) is to set booster gain from

3 to 10 with total (composite) gain at least a factor of 3 times

booster gain. Guideline (3) implies compensating the driver

as required in low composite gain configurations. Phase shift

within the loop (4) is minimized through use of booster and loop

compensation capacitors Cc and Cf when required. Typical

values are 5pF to 33pF.

Stability is the most difficult to achieve in a configuration where

driver effective gain is unity (ie; total gain = booster gain). For

this situation, Table 1 gives compensation values for optimum

square wave response with the op amp drivers listed.

SLEW RATE

The slew rate of the composite amplifier is equal to the slew

rate of the driver times the booster gain, with a maximum value

equal to the booster slew rate.

OUTPUT SWING

The maximum output voltage swing required from the driver

op amp is equal to the maximum output swing from the booster

divided by the booster gain. The Vos of the booster must also

be supplied by the driver, and should be subtracted from the

available swing range of the driver. Note also that effects of

Vos drift and booster gain accuracy should be considered when

calculating maximum available driver swing.

GENERAL

Please read Application Note 1 "General Operating Consider-

ations" which covers stability, supplies, heat sinking, mounting,

current limit, SOA interpretation, and specification interpreta-

tion. Visit www.apexmicrotech.com for design tools that help

automate tasks such as calculations for stability, internal power

dissipation, current limit; heat sink selection; Apex’s complete

Application Notes library; Technical Seminar Workbook; and

Evaluation Kits.

CURRENT LIMIT

For proper operation, the current limit resistor (R

connected as shown in the external connection diagram. The

minimum value is 0.33 with a maximum practical value of 47.

For optimum reliability the resistor value should be set as high

as possible. The value is calculated as follows:

+I

SAFE OPERATING AREA

NOTE: The output stage is protected against transient flyback.

However, for protection against sustained, high energy fly-

back, external fast-recovery diodes should be used.

COMPOSITE AMPLIFIER CONSIDERATIONS

Cascading two amplifiers within a feedback loop has many

advantages, but also requires careful consideration of several

amplifier and system parameters. The most important of these

are gain, stability, slew rate, and output swing of the driver.

Operating the booster amplifier in higher gains results in a

higher slew rate and lower output swing requirement for the

driver, but makes stability more difficult to achieve.

GAIN SET

The booster’s closed-loop gain is given by the equation

above. The composite amplifier’s closed loop gain is determined

by the feedback network, that is: –Rf/Ri (inverting) or 1+Rf/Ri

(non-inverting). The driver amplifier’s “effective gain” is equal

to the composite gain divided by the booster gain.

Example: Inverting configuration (figure 1) with

R i = 2K, R f = 60K, R g = 0 :

Av (booster) = (6.2K/3.1K) + 1 = 3

Av (composite) = 60K/2K = – 30

Av (driver) = – 30/3 = –10

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FIGURE 2. NON-INVERTING COMPOSITE AMPLIFIER.

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This data sheet has been carefully checked and is believed to be reliable, however, no responsibility is assumed for possible inaccuracies or omissions. All specifications are subject to change without notice.

PB51U REV D OCTOBER 2004 © 2004 Apex Microtechnology Corp.