BUF04

REV. 0

–9–

FUNCTIONAL DESCRIPTION

The BUF04 is a closed-loop voltage buffer based on a current

feedback architecture. Its high open-loop transimpedance, high

output current drive capability, and its low input offset voltage

makes it useful in a variety of applications, such as buffering the

inputs of sampling and flash A/D converters, audio and video

line drivers, active filters, and precision op amp hoosters.

A transistor-level equivalent circuit for the BUF04 is illustrated

in Figure 29. The input stage consists of a pair of emitter

follower transistors, Q1 and Q2, whose outputs drive a second

set of transistors, Q3 and Q4. The emitters of Q3 and Q4 are

connected together through diodes, D1 and D2, to form a low

impedance input for the feedback signal (in current mode) from

the output stage. The outputs of Q3 and Q4 are then

“mirrored” to Q5 and Q6 which form the gain stage of the

BUF04. The signal is taken from the collectors of Q5 and Q6

which drive a “Darlington-connected” output stage made up of

transistors Q7-Q10. Three R-C networks (R1–C1, R2–C2, and

R3–C3) form feed-forward paths which bypass certain sections

of the BUF04 for improved high frequency performance and

capacitive load drive capability. Since the signal conveyed

internally in the BUF04 is a current, the frequency response

and slew rate of the BUF04 are insensitive to supply voltage

variations.

12

0

–12

10k

100k

1000M

100M

10M

1M

3

6

9

–9

–6

–3

FREQUENCY – Hz

V

S

= ±15V

T

A

= +25

R

L

= 150

C

L

= 100pF

C

L

= 50pF

C

L

= 0pF

150

CL

10

BUF04

Ω

Ω

Ω

Figure 28. Bandwidth vs. Frequency

Q11

Q13

Q5

Q3

Q7

Q9

Q2

Q1

C1

C3

R3

D2

D1

Q4

R2

Q10

Q8

Q6

Q14

Q12

C2

100

Ω

20

Ω

20

Ω

Figure 29. Transistor-Level Equivalent Circuit

An interesting feature of the BUF04 architecture is the use of

“slew-enhancement” transistors, Q11–Q14. Under normal small

“OFF.” In large signals, high speed transient applications where

“brute-force” the output to follow the input. When the input

normally “OFF” state.

10

100

0%

90

50mV

10ns

50mV

INPUT

(50mV/DIV)

OUTPUT

(50mV/DIV)

Figure 25. Small-Signal Transient Response

10

100

0%

90

2V

50ns

2V

INPUT

(2V/DIV)

OUTPUT

(2V/DIV)

DLY

375.0ns

Figure 26. Large-Signal Transient Response

0.1

0.010

0.001

0.0001

20

100

1k

10k

20k

07 MAR 93 21:31:53

AUDIO PRECISION BUF04 THD+N (%) vs FREQ (Hz)

T

A

D

B

C

VS= ±15V

LPF=80kHz

: VIN = 0.775Vrms, RL= 150W

: VIN = 0.775Vrms, RL= 600W

A : VIN = 7.75Vrms, RL= 150W

B : VIN = 7.75Vrms, RL= 600W

A

B

C

D

C

C

D

Figure 27. THD + Noise vs. Amplitude