This section returns to the required gain for the amplifier design. If the DANL is examined for a typical spectrum analyzer (BW over 1kHz–100kHz), a typical DANL is found to be close to –120dBm/Hz (224nV/Hz), a noise of 10dB or higher results in approximately –110dBm/Hz or 708nV/Hz. Using this level, along with a noise of approximately 1nV/Hz for the LDO, you achieve a gain of approximately 708(V/V).

This estimate is a rough estimate for the minimum gain value for our design. This minimum gain value is much higher than the 40dB of gain that only a few analyzers offer with a preamplifier.

Figure 2-1 shows the proposed amplifier circuit.

Figure 2-1 The Proposed Amplifier Circuit

To maintain a flat gain response over the BW measurement of 10Hz–10HMz, the total necessary amplifier gain (of roughly 70dB, adjusted above the minimum value of 57dB) is carried out by two series of stages. The first stage, which is strictly in a non-inverting configuration, carries most of the gain to quickly raise the noise of the LDO (DUT) to a higher level. The second stage, which is in an inverting configuration, is used to fine-tune the total gain of the amplifier.