NE570

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7

At very high frequencies, the response of the rectifier will

fall off. The roll−off will be more pronounced at lower input

levels due to the increasing amount of gain required to switch

response for input levels of 0 dBm, −20 dBm, and −40 dBm

is shown in Figure 11. The response at all three levels is flat

to well above the audio range.

0

3

10 k

1 MEG

INPUT = 0 dBm

−20 dBm

−40 dBm

FREQUENCY (Hz)

Figure 11. Rectifier Frequency Response

vs. Input Level

VARIABLE GAIN CELL

Figure 12 is a diagram of the variable gain cell. This is a

a current mirror provides the output current.

V+

V−

NOTE:

20 k

W

280

mA

−

+

140

mA

Figure 12. Simplified

DG Cell Schematic

I2 * (I1 ) IIN) + I1 * IIN + IC2.

The op amp has thus forced a linear current swing between

This drive signal will be linear for small signals, but very

non−linear for large signals, since it is compensating for the

signal conditions.

The key to the circuit is that this same predistorted drive

two differential pairs of transistors have the same signal

applied, their collector current ratios will be identical

regardless of the magnitude of the currents. This gives us:

I

C1

I

C2

+

I

C4

I

C3

+

I

I

will yield the multiplier transfer function,

I

I

G

I

1

I

V

IN

R

2

I

G

I

1

This equation is linear and temperature−insensitive, but it

assumes ideal transistors.

4

3

2

1

0.34

−6

0

+6

4 mV

3 mV

2 mv

1 mV

INPUT LEVEL (dBm)

Figure 13.

DG Cell Distortion vs. Offset Voltage

If the transistors are not perfectly matched, a parabolic,

non−linearity is generated, which results in second

harmonic distortion. Figure 13 gives an indication of the

magnitude of the distortion caused by a given input level and

offset voltage. The distortion is linearly proportional to the

magnitude of the offset and the input level. Saturation of the

gain cell occurs at a +8.0 dBm level. At a nominal operating

level of 0 dBm, a 1.0 mV offset will yield 0.34% of second

harmonic distortion. Most circuits are somewhat better than