Understanding the P-Chip Enhancement MOSFET in Circuit Design: Amplifier or Logic Inverter?
The component in focus here is a P-Channel Enhancement MOSFET. Typically, its channel is denoted by a broken line. Its functional versatility makes it a crucial component in various circuit configurations.
Functionality as a Switch and Inverter
When used as a switch, the MOSFET's channel is enhanced when the input bias and/or signal voltage is applied to the gate. Its ability to form a digital channel under these conditions is a testament to its adaptability. Moreover, in the linear region, it can serve as an amplifier. However, to function in this mode, a DC bias is usually required to stabilize its behavior.
Determining the Functionality
This component may serve as both an amplifier or a logic NOT gate. The key difference lies in the nature of the input. If the input is an AC signal, it functions as an amplifier. Conversely, if the input is DC, it's more likely a NOT gate. An amplifier's role is to increase the amplitude of the output AC signal, while a NOT gate inverts the logic level of a digital signal.
Dual Functionality Analysis
The same circuit can be viewed in a couple of ways, depending on the input. For example, if the input signal (Vin) is small-signal AC combined with an appropriate bias DC voltage, it can operate as a class-A amplifier. However, due to its inherent limitations, this configuration is considered cheap and inefficient with notable distortion.
When the circuit lacks biasing circuitry, it often serves as a simple inverter or switch. The presence of a DC offset in the input voltage could indicate the circuit's potential for use as an amplifier. However, it's generally good practice to include feedback in biasing circuits, making this configuration improbable. The relevance of this circuit in aviation due to the common 28 V DC level in aircraft is evident from its use.
Voltage Amplifier Role
As Praveen explained, this simple circuit operates as a voltage amplifier. When the input voltage (Vin) is 0 volts, the output voltage (Vout) is 28 volts. A gradual increase in Vin to 0.2 volts results in a corresponding decrease in Vout to approximately 18 volts. Further increasing Vin to 0.5 volts leads to Vout approaching 0 volts, effectively inverting the input voltage with a small differential.
The 10k resistor serves as a load to absorb the 28V when the UJT (Unijunction Transistor) conducts at an input of 0.5 volts. This demonstrates the behavior of semiconductors, such as UJTs and transistors, and how manufacturers provide data sheets to explain output voltages with given inputs. This information is crucial for designing amplifier circuits and is known as the Characteristic Curve, a reference for such designs.