Application Information

Introduction

The CLC5509 is a two stage ultra-low noise preamplifier,

with low distortion, and externally variable input impedance.

The unusual emitter driven input stage remains stable for a

wide range of transducer source loads. The input termination

can be matched (for 50-200

Ω source matching) to a wide

range of complex loads (C

10000pF, L

multiple channel ultrasound applications requiring flexible

configurations for a variety of transmit/receive topologies. In

a typical application, the CLC5509 is connected to a single

element

of

an

ultrasound

transducer

through

a

transmit/receive switch.

Theory of Operation

The CLC5509 simplified circuit is shown in

Figure 1. For

analysis, the transmit/receive switch diode is modeled in the

circuit as a series resistance R

A piezo transducer generated, single-ended voltage signal is

applied to the emitter input of the 1st stage. The voltage sig-

nal is converted to a current (i) that is passed through a high

pass filter then restored back to a voltage signal at R

speed, low distortion, unity gain buffer, applies the signal to

the load and feedback resistor. Negative feedback from the

buffer output to the inverting input completes the signal path.

The input and output voltage can be expressed as shown:

V

V

for

α = 1

The input resistance is calculated

The current I

sets r

I

r

Combining

terms,

then

solving

for

close

loop

gain

V

Negative PSRR (V

DS101304-28

DS101304-29

FIGURE 1. Simplified Circuit

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