The analog priority amplifier shown in Figure 1 was originally designed as part of a multi-output power supply, where regulation operation is based on the voltage of the highest priority channel. Another application of this amplifier is an engine control system with electronic throttle control, where the engine needs to respond to the highest priority of multiple input commands.
Figure 1The input priority amplifier provides an output corresponding to the one of the four inputs with the largest positive value. While the circuit responds to a positive input, it responds to a negative input by reversing the direction of the diode and reconfiguring the power supply.
In this circuit, the amplifier with the maximum positive output controls the negative feedback path through a forward bias diode in the amplifier output. It forms a simple unity-gain path through R1, R2, R3, or R4 (depending on which channel has the maximum positive value) into the inverting input of the amplifier. The diode between the inverting input and output is reversed biased on the amplifier with the largest input, and the final circuit acts as a unit gain amplifier between its input and total output.
The output of an amplifier with a smaller input value is forced to change from an output value to a negative value until its feedback diode, D2 (or any corresponding amplifier's diode), is positively biased, keeping the amplifier under local closed-loop conditions. By using a 10k resistor (e.g., R1) to form a local feedback network, an amplifier with a smaller input value can be operated as a unity-gain buffer. Figure 2 shows the results when using all four channels.
Figure 2** Graph of the output of the 4-channel priority amplifier.
Exaggerate the effect when two different waveforms compete for the highest amplitude at different time periods by applying different input signals. Figure 3 shows the actual oscilloscope waveform plot of the dual-channel version of the amplifier, where channel 3 is the output (note that the zero value of channel 3 is lower on the oscilloscope screen than the zero value of channels 1 and 2).
Figure 3The oscilloscope waveform of the two-channel version of the priority amplifier channels 1 and 2 are the input signals and channel 3 is the output. (Note that the zero value of channel 3 is lower on the oscilloscope screen than the zero value of channels 1 and 2.) ）
Although the circuit is configured to respond to a positive voltage, it is only necessary to reverse the direction of the diode connection and set the supply voltage appropriately to respond to a negative voltage instead.
The circuit shown in the diagram uses a Microchip MCP6V51 2 4 op amp, but there are many more op amps to choose from. The following factors need to be considered when choosing an op amp:
1.Multiple op amps, such as four op amps (and multiplier the number of op amps, depending on the final number of lines).
2.Most applications typically require the op amp's common-mode range, which includes the negative power rail (typically ground) of the op amp. In some cases, an amplifier with a rail-to-rail common-mode range may be required.
3.The voltage rating required for an op amp is obviously determined by the size of the sensor or input signal as well as the output signal requirements.
4.For this circuit, unity-gain stability is critical. When the output goes into capacitive load, additional compensation of the op amp may be required to maintain stability.
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