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

 Lecture for Measurement

Posted 2019/7/19

Voltage amplifier ("voltage to voltage amplifier")

NF voltage amplifier

2. Circuit design featuring use of negative-feedback technology

The SA series is configured by gathering various technologies for each model.

For example, while SA-250F6 has an input impedance of 50 Ω, its equivalent input noise voltage density is lower than thermal noise due to resistance of 50 Ω; in other words, it seems that SA-250F6 has the feature of negating the physical low. However, SA-250F6 is not a device that transcends the physical law; it was designed according to the existing physics (electrical and electronic engineering) theory. It was designed by using a technique called “active input impedance”, which plays an important role in supporting low noise (Figure).


A simplified version of an amplifier with a typical input impedance of 50 Ω is shown in Figure (a). As for this method, 50 Ω thermal noise connected to the input is mixed with the signal, amplified, and output. For that reason, it is impossible for thermal noise to fall below that due to 50 Ω resistance, even in the case of an ideal amplifier without internal noise. The method for breaking through this “50 Ω wall” is the active input impedance method shown in Figure (b). Instead of removing the terminating resistors, input impedance is created with a negative-feedback (NF) resistor.

The SA-250F6 is an amplifier with a gain of 100 times (40 dB). Applying the active input impedance method compresses the influence of thermal noise to 1/10 or less compared with method (a), which uses a 50 Ω termination resistor as is.

This active input impedance method is essential for achieving an equivalent input noise voltage density that is lower than the 50 Ω thermal noise while maintaining an input impedance of 50 Ω.

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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