Name: mosfet gain and biasing
Uploaded: 2024-07-08T13:20:42.000Z
Duration: 1 min 17 s
Channel: JoVE

Der MOSFET arbeitet als spannungsgesteuerte Stromquelle in seinem aktiven Bereich, in dem die Gate-to-Source-Spannung den Drain-Strom regelt.
Dieser Transkonduktanz-MOSFET-Verstärker kann in einen Spannungsverstärker umgewandelt werden, indem der Ausgangsstrom durch einen Widerstand geleitet wird.
Die Ausgangsspannung erhält man, indem der Spannungsabfall über den Lastwiderstand von der Versorgungsspannung abgezogen wird. Dies führt zu einer invertierten Version der Ausgangsspannung, die um die Versorgungsspannung verschoben wird.
Das Spannungsübertragungsdiagramm des Verstärkers veranschaulicht die Beziehung zwischen dem Ausgangsdrain und der Eingangsgate-Spannung.
Er hebt den aktiven Bereich des Verstärkers hervor, mit einer steilen Steigung für maximale Verstärkung, aber einer nichtlinearen Drain-Spannung.
Das Anlegen einer Gate-to-Source-Gleichspannungsvorspannung am Ruhepunkt innerhalb des aktiven Bereichs führt in der Regel zu einer nahezu linearen Verstärkung des MOSFET.
Wenn ein kleines, zeitlich veränderliches Signal an den Verstärker angelegt wird, überlagert es sich mit der DC-Vorspannung um den Q-Punkt.
Der MOSFET arbeitet innerhalb eines kurzen, nahezu linearen Segments der Kennlinie, was zu einer verstärkten Ausgangs-Drain-Spannung führt.

transistoren

Der MOSFET fungiert, wenn er in seinem aktiven Bereich betrieben wird, als spannungsgesteuerte Stromquelle. In diesem Bereich steuert die Gate-Source-Spannung den Drain-Strom. Dieses Prinzip liegt dem Betrieb des Transkonduktanz-MOSFET-Verstärkers zugrunde. Der Ausgangsstrom wird durch einen Lastwiderstand geleitet, um diesen Verstärker in einen Spannungsverstärker umzuwandeln. Die Ausgangsspannung wird dann erhalten, indem der Spannungsabfall über dem Lastwiderstand von der Versorgungsspannung abgezogen wird. Dieser Prozess führt zu einer invertierten Ausgangsspannung, die um die Versorgungsspannung verschoben ist.

Das Diagramm der Spannungsübertragungskennlinie des Verstärkers veranschaulicht die Beziehung zwischen der Ausgangs-Drain-Spannung und der Eingangs-Gate-Spannung. Dieses Diagramm ist entscheidend für das Verständnis des Verhaltens des Verstärkers. Es hebt den aktiven Bereich des Verstärkers hervor, in dem die Steigung der Kurve steil ist, was auf maximale Verstärkung hinweist. Dieser Bereich ist jedoch auch in Bezug auf die Drain-Spannung nicht linear.

Eine Gleichspannungsvorspannung wird an die Gate-Source-Verbindung angelegt, um eine nahezu lineare Verstärkung zu erreichen, wodurch der MOSFET an einem Ruhepunkt (Q-Punkt) innerhalb des aktiven Bereichs positioniert wird. Diese Vorspannung stellt sicher, dass der MOSFET in einem Bereich arbeitet, in dem sein Verhalten annähernd linear ist. Wenn dieser Gleichstromvorspannung ein kleines, zeitlich variierendes Signal überlagert wird, bewirkt dies, dass der MOSFET um den Q-Punkt herum arbeitet. Der MOSFET arbeitet daher innerhalb eines kurzen, nahezu linearen Abschnitts seiner Kennlinie, was zu einer verstärkten Ausgangsdrainspannung führt.

In praktischen Anwendungen ermöglicht dieser Aufbau dem MOSFET-Verstärker, kleine Wechselstromsignale effektiv zu verstärken. Die Verstärkung tritt auf, weil das kleine Eingangssignal die Gate-Spannung um den Q-Punkt herum moduliert, was proportionale Variationen im Drain-Strom verursacht. Diese Variationen werden in eine größere Ausgangsspannung über dem Lastwiderstand umgesetzt, wodurch eine Verstärkung erreicht wird.

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Der MOSFET arbeitet als spannungsgesteuerte Stromquelle in seinem aktiven Bereich, in dem die Gate-to-Source-Spannung den Drain-Strom regelt.
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MOSFET Amplifiers

mosfet gain and biasing

MOSFET gain and biasing shape how a MOSFET amplifier works. When a MOSFET operates in its active region, it acts like a voltage-controlled current source. The gate-to-source voltage sets the drain current, and that behavior supports transconductance amplification.

The output current then flows through a load resistor. That load resistor turns the circuit into a voltage amplifier. The output voltage is found by subtracting the voltage drop across the load resistor from the supply voltage, so the result is inverted and shifted by the supply voltage.

A voltage-transfer characteristic plot shows how the output drain voltage changes with the input gate voltage. This curve helps explain the amplifier’s behavior. The active region has a steep slope and gives maximum gain, but it is also nonlinear in drain voltage.

To make the amplification nearly linear, a DC bias voltage is applied to the gate-to-source junction. This sets the MOSFET at a quiescent point, or Q-point, inside the active region. A small time-varying signal can then ride on top of the DC bias and keep the MOSFET operating around that point.

Around the Q-point, the MOSFET uses only a short, nearly linear part of its characteristic curve. That lets it amplify small AC signals effectively. Small changes in gate voltage cause proportional changes in drain current, and the load resistor turns those current changes into a larger output drain voltage.

MOSFET Gain and Biasing

bipolar junction transistor

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MOSFET Structure and Operating Regions

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mosfet amplifiers

MOSFET gain and biasing shape how a MOSFET amplifier works. When a MOSFET operates in its active region, it acts like a voltage-controlled current source. The gate-to-source voltage sets the drain current, and that behavior supports transconductance amplification. The output current then flows through a load resistor. That load resistor turns the circuit into a voltage amplifier. The output voltage is found by subtracting the voltage drop across the load resistor from the supply voltage, so...

small-signal analysis of mosfet amplifiers

small-signal-analysis-of-mosfet-amplifiers

MOSFET Amplifier Gain and Signal Distortion

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key terms and concepts

Learn how MOSFET biasing sets a Q-point, how the load resistor creates voltage gain, and why the output signal is inverted in the active region.

MOSFET Gain and Biasing | JoVE K12

JoVE Core Electrical Engineering offers easy-to-understand animated videos that cover fundamental concepts of electric circuits along with circuit analysis techniques. It incorporates first-order and second-order circuits using operational amplifiers and circuit transformation concepts. The inclusion of practical design examples and real-world applications through scientist-in-action videos adds significant educational value, making complex concepts accessible.

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MOSFET Gain and Biasing

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