Typical Performance Characteristics

Automatic Level Control

(ALC) Response Characteristic

Preamp Input Noise Voltage

Preamp Input Noise Current

TLH7894 – 5

Application Hints

PREAMPLIFIERS

(Figure 5)

There are 2 identical preamplifiers with 1 common output

pin on the IC One amplifies low level inputs such as a mi-

crophone in the record mode and another amplifies the sig-

nal from the playback head in the playback mode The am-

plifiers use a common capacitor C6 to set the low frequen-

cy pole of the closed loop responses On the playback am-

plifier the collector of the input device is made available so

that an external low noise device can be connected in crit-

ical applications When using an external low noise transis-

tor pins 17 and 18 of the IC are shorted together to ensure

that the internal input transistor is turned OFF and the exter-

nal transistor’s collector is tied to pin 19 The input and

feedback connections are now made to the external input

transistor The amplifiers are stable for all gains above 5

and have a typical open loop gain of 100 dB R8 and R9

enable C6 to be quickly charged and set the DC gain Inter-

nal biasing provides a DC voltage independent of tempera-

ture at pin 17 so that the preamplifier DC output will remain

relatively constant with temperature Supply decoupling is

provided by an internal regulator Additional decoupling can

be added for the input stages by increasing the size of the

capacitor on pin 20 of the IC A fast charging circuit is con-

nected to the preamplifiers’ input capacitors (pins 16 and

17) to decrease the turn-ON time Larger input capacitors

decrease the noise by reducing the source impedance at

lower frequencies where 1f noise current produces an in-

put noise voltage The input resistance of the preamplifiers

is typically 50 kX

TLH7894 – 6

Quiescent DC Output Voltage

R8R9

AC Voltage Gain

R3

R2

a

1

TLH7894 – 7

FIGURE 5 Preamplifier

6