REV. B

OP470

–14–

SQUELCH AMPLIFIER

The circuit of Figure 17 is a simple squelch amplifier that uses a

FET switch to cut off the output when the input signal falls

below a preset limit.

The input signal is sampled by a peak detector with a time

constant set by C1 and R6. When the output of the peak detector

(Vp), falls below the threshold voltage, (VTH), set by R8, the

comparator formed by op amp C switches from V– to V+. This

drives the gate of the N-channel FET high, turning it ON, re-

ducing the gain of the inverting amplifier formed by op amp A

to zero.

1/4

OP470E

A

R1

2k

R2

10k

2N5434

R5

100k

1/4

OP470E

B

R3

2k

R4

10k

D1

1N4148

C1

1 F

R6

1M

1/4

OP470E

C

R4

10M

D2

1N4148

R7

10k

+

C2

10 F

V+

R6

10k

= 1 SECOND

Figure 17. Squelch Amplifier

FIVE-BAND LOW-NOISE STEREO GRAPHIC EQUALIZER

The graphic equalizer circuit shown in Figure 18 provides 15 dB

of boost or cut over a 5-band range. Signal-to-noise ratio over a

20 kHz bandwidth is better than 100 dB referred to a 3 V rms

input. Larger inductors can be replaced by active inductors but

this reduces the signal-to-noise ratio.

R1

47k

1/4

OP470E

R13

3.3k

+

R14

100

R4

1k

60Hz

R2

3.3k

1/4

OP470E

C1

0.47 F

C2

6.8 F

TANTALUM

L1

1H

R3

680

+

R4

1k

200Hz

C3

1 F

TANTALUM

L2

1H

R5

680

+

R4

1k

800Hz

C4

0.22 F

TANTALUM

L3

1H

R7

680

+

R4

1k

3kHz

C5

0.047 F

TANTALUM

L4

1H

R9

680

+

R4

1k

10kHz

C6

0.022 F

TANTALUM

L5

1H

R11

680

Figure 18. Five-Band Low Noise Graphic Equalizer