OP250/OP450

REV. 0

–13–

1/2

OP250

20

50k

270 F

LEFT

HEADPHONE

10 F

50k

50k

100k

10 F

LEFT

INPUT

+V + 5V

1/2

OP250

20

50k

270 F

RIGHT

HEADPHONE

10 F

50k

50k

100k

10 F

RIGHT

INPUT

+V

+V + 5V

1 F/0.1 F

Figure 37. A Single-Supply Stereo Headphone Driver

FREQUENCY – Hz

1

0.1

0.001

20

20k

100

1k

0.01

10k

Figure 38. THD vs. Frequency

Headphone Driver

The audio signal is coupled into each input through a 10

µF ca-

pacitor. This large value insures the resulting high pass filter

cutoff is below 20 Hz, preserving full audio fidelity. If the input

already has the proper dc bias, then the coupling capacitor and

biasing resistors are not required. A 270

µF capacitor is used at

the output to couple the amplifier to the headphone speaker.

This value is much larger than the input capacitor because of

the low impedance of the headphones, which can range from

32

Ω to 600 Ω or more. An additional 20 Ω resistor is used in

series with the output capacitor to protect the op amp’s output

in the event the output accidentally becomes shorted to ground.

Direct Access Arrangement for Modems

Figure 39 illustrates a +5 V transmit/receive telephone line inter-

face for 600

Ω systems. It allows full duplex transmission of sig-

nals on a transformer coupled 600

Ω line in a differential manner.

Amplifier A1 provides gain which can be adjusted to meet the

modem output drive requirements. Both A1 and A2 are config-

ured so as to apply the largest possible signal on a single supply to

the transformer. Because of the OP450’s high output current

drive and low dropout voltages, the largest signal available on a

single +5 V supply is approximately 4.5 V p-p into a 600

Ω trans-

mission system. Amplifier A3 is configured as a difference ampli-

fier for two reasons: (1) It prevents the transmit signal from

interfering with the receive signal and (2) it extracts the receive

signal from the transmission line for amplification by A4. Ampli-

fier A4’s gain can be adjusted in the same manner as A1’s to meet

the modem’s input signal requirements. Standard resistor values

permit the use of SIP (Single In-line Package) format resistor ar-

rays. Couple this with the OP450 14-lead TSSOP or SOIC foot-

print and this circuit offers a compact, cost-effective solution.

6.2V

6.2V

TRANSMIT

TXA

RECEIVE

RXA

C1

0.1 F

R1

10k

R2

9.09k

2k

P1

TX GAIN

ADJUST

A1

A2

A3

A4

A1, A2, A3, A4 = 1/4 OP450

R3

360

1:1

T1

TO TELEPHONE

LINE

1

2

3

7

6

5

2

3

1

6

5

7

10 F

R7

10k

R8

10k

R5

10k

R6

10k

R9

10k

R14

14.3k

R10

10k

R11

10k

R12

10k

R13

10k

C2

0.1 F

P2

RX GAIN

ADJUST

2k

600

+5V DC

MIDCOM

671-8005

Figure 39. A Single-Supply Direct Access Arrangement for

Modems