# Driving the future of technology : Original solution in measurement and control

## Lecture for Measurement

Posted 2019/7/19

### Voltage amplifier ("voltage to voltage amplifier")

#### 4. Input impedance of the preamplifier

Among the preamplifiers in our lineup, SA-250F6 has the lowest equivalent input noise voltage density (0.25 nV/√Hz typ.).

Even so, does SA-250F6 provide the best performance in any application? The answer to that question is “no” because SA-250F6 is only suitable for specific uses. For that reason, we offer a lineup that makes it possible to select a preamplifier that matches each application.

The signal source (sensor) always has output impedance. In general, it is desirable that the input impedance of the preamplifier is high; however, the output impedance of the signal source is low, and, in reality, they are both finite values and do not become infinity or zero.

SA-250F6 has an input impedance of only 50 Ω because it is designed to be optimized for a 50 Ω system.

For that reason, it is not the best one to amplify a signal output from a high-output-impedance signal source like a piezoelectric element. If the output impedance is as high as 1 kΩ, the signal is attenuated to about 1/20 at the input terminal of SA-250F6 (Figure).

In the case of such high output impedance, it is better to select a model amplifier with high input impedance.

For example, SA-220F5 is a preamplifier with input impedance of 1 MΩ. Even sensors with output impedance of 1 kΩ can be used with it without concern for attenuation.

Figure

Related Product

Extremely low noise amplifier SA Series

Extremely low noise, excellent stability, wideband preamplifier for detection of extremely small signals

Low noise
• Adopted our exclusive circuit applying negative feedback technology
SA-250F6: at input impedance of 50 Ω, noise figure of 0.6 dB and equivalent input noise voltage density of 0.25 nV/√Hz
•  Wideband
• A wide frequency band from low-frequency signals including DC (SA-200F3/240F5/410F3/440F5) to high-frequency, high-speed signals (SA-250F6/251F6) exceeding 100 MHz is covered.
•  Input mode according to signal source
• Single-ended (single-wire-grounded) input, single-ended FET input, differential input, and differential FET input

Related keywords :  Ultra low noise amplifier, LNA

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