In this document, we introduce two technologies, bias stabilization technology and active input impedance to achieve low noise characteristics employed in SA-230F5 and SA-430F5.

### Bias-stabilization technology

The bias condition is temperature dependant. To solve this problem, inserting a resistor in the emitter of the input transistor is a solution, but that resistor would generate noise. The bias stabilization technology can stabilize the bias without using the resistor.

Fig. 1 shows the basic ampliﬁcation circuitry of SA-230F5. By DC servo of a diﬀerential integrator, the collector current *Ic* of the input transistor is *IC = VREF / RC.*

The transconductance *gm* of the circuit in Fig. 1 is

Where k is the Boltzmann constant, *q* is the elementary charge, and *T* is the absolute temperature.

Gain *A* is

According to this equation, the gain is inversely proportional to the absolute temperature. If *VREF* is proportional to absolute temperature, the gain *A* will not depend on temperature. It is possible to make *VREF* proportional to temperature by using a dedicated *IC* or a resistor with a temperature coeﬃcient.

if *VREF* = *α・T*, the gain *A* is

Then, it will not be temperature dependant.

**Figure 1. **Basic Amplification Circuitry of SA-230F5

### Active input impedance

The active input impedance can be matched in a wide frequency range rather than at a point in conjugate matching. In addition, the noise ﬁgure which could be up to 3 dB by a termination resistor, can be less than 3 dB.

The input impedance of the circuit whose input and output of the inverting ampliﬁer are connected by resistor *Rf* as shown in Fig. 2 can be calculated as *Zin = Rf / (1 + A)*.

This value depends on the gain.

Because a stable gain can be achieved by the bias stabilization technology mentioned above, the stable input impedance can be kept.

**Figure 2.** Active Input Impedance

Utilizing these two technologies, the SA-230F5 achieves ultra low-noise characteristics in a wide range as shown in Fig 3.

**Figure 3.** Noise Figure of SA-230F5