Utilise audio amps for voltage splitting (Part 4)
Reducing the noise from the voltage splitters
The analogue VSs based on linear audio amplifiers do not create a lot of noise in the output voltages. The PAAs and OAs are developed for audio applications and a lot of efforts are made to reduce the internal noise, which is amplified by the IC. Nevertheless we may be in need to reduce the output noise.
That can be done with RC, RL, RLC and LC filters or with additional low-noise regulators. The problem with the passive filters is that they are reactive loads and not all audio amplifiers are stable with them. Sometimes we may need to connect a resistor or a resistor in parallel with a high-current inductor in series of the output of the PAA.
Figure 17 presents how to connect three LC filters to the VS. The filters reduce the noise from the input voltage and the noise produced by PAA.
Figure 17: Connecting LC filters to a voltage splitter around PAA.
In order to reduce the noise of the PAA we can do at least the following:
We can connect capacitor between the inverting and non-inverting inputs of the PAA,
We can reduce the band width of the PAA
We should not to connect the inputs of the PAA directly to the ground lines or to the power supply lines
Other common practices for noise reduction in the audio amplifiers and power supplies are also applicable.
The stability of the VS with reactive and changing loads
The parameters of PAAs and OAs are optimised for certain applications and loads. We need to consider the DC stability, the AC stability, the stability during the transitional processes, the stability when the load is switching or slowly varied or modulated, etc.
The applications of the audio amplifying circuits in VSs are not typical applications. Usually in these applications the audio amplifier works with low DC gain, with high capacitive, inductive or mixed loads, with switching and non-symmetrical loads, and the AC gains of the IC can be different, etc. These particularities of the VS applications can create a lot of problems.
If we use the VS in radio transmitters, ultrasound transmitters or in powerful low-frequency generators or similar oscillating and emitting devices we may found a tendency to modulate the output voltage and the IC can become unstable. In these applications the loads can return current to the output of the VSs or can apply reverse voltages.