# Low-Noise Programmable Voltage Source
## 4 Noise Performances Comparison with Solid-State Voltage Sources  
To better appreciate the noise performances of the proposed voltage source, we can compare it to what can be obtained by resorting to a commercial solid-state very low-noise linear regulator. For this purpose, we resorted to the low drop out (LDO) regulator model LT3045 that was chosen as the one with the lowest reported level of output noise. Its output voltage can be set using a single resistor \(R_{\mathrm{SET}}\), as in Figure 7. This regulator is characterized by s voltage noise of 0.8 \(\mu\)V\({}_{\mathrm{RMS}}\) in the frequency range from 10 Hz to 100 kHz and a noise density of about 2 nV/VHz (10 kHz). In order to obtain a lower noise level and a higher output current, a few LT3045 regulators can be paralleled. In this regulator, the output noise voltage mainly depends on the capacity of \(C_{\mathrm{SET}}\). This capacitor shunts the thermal noise generated in resistor \(R_{\mathrm{SET}}\) and filters out the noise from the internal current source. The circuit in Figure 7 used for testing was built according to the manufacturer application notes. For the capacity of \(C_{\mathrm{SET}}\), two paralleled ceramic capacitor X5 Rs were used, providing a larger capacity than the one reported in the datasheet. The regulator was supplied using a lead acid battery for \(V_{\mathrm{IN}}\) in Figure 7.  
The measured \(VSD\) at the output of the circuit in Figure 7 is reported in Figure 8 for \(V_{out}\) = 50 mV and \(V_{out}\) = 1 V. A 1 k\(\Omega\) wire-wound resistor was used as a load. Figure 6: Voltage error vs. time in stabilized state of voltage source with the 2 SK3557-6 JFET follower. The results are comparable with the ones reported in the catalogue. It is clear, especially in the low frequency region (below 100 Hz), that the level of noise that can be obtained with our proposed design is much lower than what can be obtained with a circuit based on a low-noise solid-state regulator. On the other hand, the circuit in Figure 7 is extremely simple, and the level of noise obtained at the output is comparable to the ones than can be obtained by resorting to commercial programmable voltage sources. There are cases, however, when the availability of a very low-noise voltage source (LNVS) such as the one we propose is essential in order to obtain correct results in the characterization of electron devices. As an example, we employed both the LNVS and a source based on the LT3045 for the characterization of the noise in a type II superlattice photodetector (T2 SL) [41]. The measurement setup for noise measurements is reported in Figure 9.  
Figure 8: _VSD_ characteristics of the voltage source in Figure 7 for different output voltages (red and blue lines) and noise background (black line). Figure 7: Voltage source with linear regulator LT3045 [40]. The setup in Figure 9 includes a low-noise trans-resistance amplifier (\(IC1\)) where the \"voltage source\" block is used to bias the device under test (DUT), the AC coupling filter (\(CIR1\)) and a low-noise voltage amplifier (\(IC2\)) followed by the acquisition and elaboration blocks in order to perform spectral estimation. The results of noise measurements for different bias voltage values when the LNVS and the LT3045-based source are used for biasing the T2 SL are reported in Figure 10.  
The curve labeled 0 V was obtained with node B grounded and represents the noise of the photodetector and the background noise of the system, excluding the noise coming from the voltage source. As it can be noticed, when the LNVS (with 2 SK3557-6 JFETs at the output) is used for biasing the T2 SL, pure flicker noise is obtained when the noise is large enough to rise above the background noise. When the voltage source based on LT3045 is used, instead, we can clearly notice the deviation in the measured noise due to the large noise introduced by the source in the frequency ranges below 10 Hz.