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.
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