Name: bjt amplifier gain and input resistance
Uploaded: 2025-05-22T09:06:20.000Z
Duration: 1 min 21 s
Channel: JoVE

Stellen Sie sich eine BJT-Transistorverstärkerschaltung vor, die in ihrem aktiven Bereich arbeitet, um eine lineare Verstärkung zu gewährleisten.
Die momentane Gesamtspannung des Basis-Emitters, die die Gleichspannung und das Eingangssignal kombiniert, ergibt einen momentanen Kollektorstrom.
Der Exponentialterm mit der Basis-Emitter-Spannung wird durch die Kleinsignal-Näherung vereinfacht, bei der das Eingangssignal deutlich kleiner ist als die thermische Spannung.
Der Kollektorstrom setzt sich also aus zwei Komponenten zusammen: dem DC-Bias-Strom und der Signalkomponente.
Das Verhältnis des Signalanteils zur variierenden Basis-Emitter-Spannung wird als BJT-Transkonduktanz bezeichnet.
Dabei ist die Transkonduktanz des BJT direkt proportional zum DC-Bias-Strom, der durch die Steigung der Tangente relativ zur Kennlinie am Bias-Punkt bestimmt wird.
Die Spannungsverstärkung entspricht also dem negativen Produkt aus der Transkonduktanz und dem Lastwiderstand. Das negative Vorzeichen zeigt eine Phasenverschiebung von 180 Grad zwischen dem verstärkten und dem Eingangssignal an.
Bei der Untersuchung des Senders wird der Kleinsignal-Basis-Sender-Widerstand ermittelt, indem die thermische Spannung durch den jeweiligen DC-Bias-Strom dividiert wird.

kleinsignalanalyse von bjt-verstärkern

Die Kleinsignalanalyse ist ein grundlegender Ansatz in der Elektronik, um zu verstehen, wie ein Bipolar Junction Transistor (BJT)-Verstärker Signale verarbeitet. Im aktiven Bereich ist der BJT für eine lineare Verstärkung ausgelegt. Das Verhalten des Transistors unter diesen Bedingungen wird durch seine momentane Basis-Emitter-Spannung V_BE, eine Summe aus der Gleichstromvorspannung V_BE und einem kleinen Wechselstromsignal V_BE bestimmt, was den Kollektorstrom i_C ergibt. Hier hat der Kollektorstrom eine Gleichstromkomponente und eine Wechselstromkomponente.

<img alt="Equation 1" src="/files/ftp_upload/15731/15731_Equation_1.svg" style="height: 72px;">

Hier ist V_T die thermische Spannung.

Wenn das Wechselstrom-Eingangssignal V_BE deutlich kleiner als V_T ist, kann eine Kleinsignalnäherung verwendet werden, um den Ausdruck für die Wechselstromkomponente des Kollektorstroms zu erhalten. Die Wechselstromkomponente des Kollektorstroms ist das Verhältnis des Produkts der Gleichstromkomponente des Kollektors mit zeitabhängiger Eingangsspannung zur thermischen Spannung.

<img alt="Equation 2" src="/files/ftp_upload/15731/15731_Equation_2.svg" style="height: 54px;">

Das Verhältnis der Wechselstromkomponente des Kollektorstroms zum Eingangssignal wird als Transkonduktanz bezeichnet. Die Spannungsverstärkung des BJT ist eine Funktion der Transkonduktanz des BJT und des Lastwiderstands.

<img alt="Equation 3" src="/files/ftp_upload/15731/15731_Equation_3.svg" style="height: 88px;">

Hier bedeutet das negative Vorzeichen eine 180-Grad-Phasenverschiebung zwischen Eingangs- und Ausgangssignalen.

An der Basis des BJT ist der Kleinsignal-Basis-Emitter-Widerstand rπ entscheidend für Überlegungen zur Eingangsimpedanz und wird bestimmt durch:

<img alt="Equation 4" src="/files/ftp_upload/15731/15731_Equation_4.svg" style="height: 60px;">

Dieser Widerstand beeinflusst die Reaktion des Transistors auf Eingangssignale und ist umgekehrt proportional zum Gleichstrom-Vorspannungsstrom.

Durch Kleinsignalanalyse kann die detaillierte Leistung von BJT-Verstärkern mit kleinen Eingangsvariationen präzise charakterisiert und im elektronischen Schaltungsdesign genutzt werden.

Kleinsignalanalyse von BJT-Verstärkern

Stellen Sie sich eine BJT-Transistorverstärkerschaltung vor, die in ihrem aktiven Bereich arbeitet, um eine lineare Verstärkung zu gewährleisten.
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Small-Signal Analysis of BJT Amplifiers

bjt amplifier gain and input resistance

BJT amplifier gain and input resistance can be analyzed with a small-signal model. In the active region, a bipolar junction transistor is used for linear amplification. Its base-emitter voltage includes a DC bias and a small AC signal. That input produces a collector current with both DC and AC parts.

When the AC input signal is much smaller than the thermal voltage VT, the small-signal approximation applies. This makes it possible to find the AC collector current from the DC collector current and the changing input voltage. The ratio of AC collector current to input signal is called transconductance. It shows how strongly the transistor converts a small voltage change into a current change.

The voltage gain of a BJT amplifier depends on the transconductance and the load resistance. The negative sign in the gain expression means the output is 180 degrees out of phase with the input. Another important small-signal feature is the base-emitter resistance rπ. This resistance matters for input impedance, and it is inversely proportional to the DC bias current. Small-signal analysis helps describe how BJT amplifiers respond to small input changes in circuit design.

BJT Amplifier Gain and Input Resistance

bipolar junction transistor

BJT Structure and Carrier Flow

Bipolar junction transistors are important parts of electronic circuits. They help make amplifiers, memories, and microprocessors work. A BJT can be built as either NPN or PNP, depending on its doping pattern, which means the type of impurities added to the material. A BJT has three layers: the emitter, base, and collector. It also has two p-n junctions, so the device has a sandwich-like structure. The emitter is heavily doped so it can inject charge carriers into the base. The base is very...

configurations of bjt

BJT Amplifier Modes and Uses

Bipolar Junction Transistor (BJT) configurations are defined by which terminal is shared by the input and output circuits. The shared terminal can be the base, emitter, or collector. Each setup has different electrical behavior and fits different circuit needs. In the common base configuration, the base is the common terminal. This setup has high voltage gain, so it can work well in single-stage amplifier circuits such as microphone pre-amplifiers. It also reduces the input current signal...

working principle of bjt

PNP BJT Current Flow in Active Mode

PNP bipolar junction transistors control current flow by moving charge carriers between the emitter, base, and collector. In a common-base setup, the transistor can amplify or switch electronic signals when it operates in active mode. In a PNP transistor, the emitter is heavily doped with holes, which are positive charge carriers. The base is lightly doped with electrons, which are negative carriers. A forward bias across the emitter-base junction lowers the potential barrier and lets holes...

characteristics of bjt

BJT Current-Voltage Curves in Common Emitter

The Bipolar Junction Transistor (BJT) in a common-emitter setup is studied by measuring its current-voltage curves. These measurements show how the transistor responds when voltage and current are changed in a controlled way. They are key to understanding how a BJT works in electronic circuits. The input characteristics are found by varying the base-emitter voltage while keeping the collector-emitter voltage constant. This test shows a Shockley-type relationship between the collector current...

modes of operations of bjt

BJT Biasing: Active to Inverted Mode

A bipolar junction transistor (BJT) works in four modes that depend on how its two junctions are biased. Those modes are active, saturation, cut-off, and inverted. The biasing pattern controls whether the transistor amplifies current or acts like a switch. Active mode is the most common mode for amplification. In this mode, the emitter-base junction is forward-biased and the base-collector junction is reverse-biased. Electrons move from the emitter into the base, while the collector blocks...

frequency response of bjt

BJT Amplifier Response at Low and High Freq

A BJT amplifier’s response changes with frequency in a common-emitter circuit. Its behavior at low frequencies and high frequencies depends on the circuit’s equivalent model. These models help explain how the transistor handles AC signals. At low frequencies, the DC bias point sets the transistor’s operation. The emitter-base voltage, base current, and collector current define this bias point. The load line also shapes how the amplifier works, using the applied voltage and load resistance. <br...

cut-off frequency of bjt

BJT Frequency Limits and Gain Drop

BJT frequency limits show when a bipolar junction transistor stops keeping a steady gain. These limits matter because they mark the shift from the pass band to the stop band in an electronic signal. They help explain why a transistor may amplify one range of frequencies well and weaken signals at higher frequencies. The alpha cut-off frequency applies to the common-base configuration. At low frequencies, the current gain alpha stays stable. Beyond this upper-frequency limit, alpha drops and...

switching of bjt

BJT as a Digital On-Off Switch

Bipolar junction transistors (BJTs) can work as digital on-off switches in electronic circuits. In this mode, the transistor moves between cut-off and saturation. These two states match the off and on positions of an ideal switch. In cut-off mode, both the emitter-base junction and the collector-base junction are reverse-biased. This blocks current flow through the transistor and keeps it off. Circuits use this state when no signal transmission is needed, which helps maintain a low-power...

bjt amplifiers

Common-Emitter BJT Voltage Gain

Bipolar junction transistors, or BJTs, can act as amplifier parts when they work in the active region. In this mode, a small change in base-emitter voltage controls a larger collector current. That makes the transistor act like a voltage-controlled current source and lets it boost a weak signal into a stronger one. A common-emitter BJT amplifier uses that current change to create voltage gain. The output current flows through a load resistor, and the voltage drop across that resistor sets the...

small-signal analysis of bjt amplifiers

BJT amplifier gain and input resistance can be analyzed with a small-signal model. In the active region, a bipolar junction transistor is used for linear amplification. Its base-emitter voltage includes a DC bias and a small AC signal. That input produces a collector current with both DC and AC parts. When the AC input signal is much smaller than the thermal voltage VT, the small-signal approximation applies. This makes it possible to find the AC collector current from the DC collector current...

field effect transistor

Field Effect Transistor Types and Uses

Field-effect transistors, or FETs, are important parts of electronic circuits. They have three terminals: gate, drain, and source. FETs are unipolar devices, which means they use either electrons or holes as charge carriers. That is different from bipolar transistors, which use both types of carriers. A FET controls the flow of charge from the source to the drain through a channel. The voltage difference between the gate and source controls how well that channel conducts. This makes the device...

characteristics of jfet

JFET Output Regions and Current Control

JFET output characteristics show how drain current changes with drain-source voltage and gate-source voltage. In a junction field effect transistor, the gate-source voltage controls the drain current. When the drain and gate voltages are set to zero, the device is in equilibrium and no net current flows. With the gate voltage held at zero, the drain current rises linearly as the source-drain voltage increases. This linear part is called the Ohmic region. In this region, the JFET acts like a...

biasing of fet

JFET Gate-Source Bias and Current Control

JFET gate-source bias controls current flow in a junction field effect transistor. A JFET uses one carrier type and comes in N-channel or P-channel forms. In both cases, the channel is surrounded by PN junctions, and those junctions help control how current moves through the device. In an N-channel JFET, N-type material forms the channel on a P-type substrate. The gate is made of P-type material. Biasing this device means applying a negative voltage to the gate relative to the source. That...

mos capacitor

MOS Capacitor Voltage and Charge

A MOS capacitor uses voltage to control charge in a semiconductor. It is a key structure in semiconductor device technology and in the fabrication of integrated circuits and MOSFETs, or metal-oxide-semiconductor field-effect transistors. The structure has three layers. A metal gate sits on top and is usually made from a highly conductive material such as aluminum or polysilicon. Below it is a thin insulating oxide layer, usually silicon dioxide (SiO2), and below that is a semiconductor...

mosfet

MOSFET Current Control in Electronics

The MOSFET is a semiconductor device that controls electrical current in modern electronics. It is also called an IGFET or MISFET. The three main terminals are the source, drain, and gate. MOSFETs are classified as n-channel or p-channel devices. The type depends on the doping of the substrate and the source or drain regions. In an n-MOSFET, the source and drain are n-type regions separated from a p-type substrate by reverse-biased p-n diodes. The gate is built from a metal plate over an...

characteristics of mosfet

MOSFET Structure and Operating Regions

MOSFETs, or metal-oxide-semiconductor field-effect transistors, are important parts of electronic circuits. They are used to amplify and switch signals. Their behavior depends on several design features, including channel size, oxide thickness, junction depth, and substrate doping. Channel length and channel width shape how much current a MOSFET can carry and how fast it can switch. Shorter channels usually allow faster operation. The oxide thickness also matters. This silicon dioxide layer...

mosfet: enhancement mode

MOSFET Switching in Enhancement Mode

Enhancement-mode MOSFETs are MOSFETs that work as controlled electronic switches. They belong to the metal-oxide semiconductor field-effect transistor family. These devices come in two forms: n-channel and p-channel, and each one is used for a different polarity of operation. At zero gate-source voltage, or Vgs, an enhancement-mode MOSFET is usually non-conductive. In that default off state, no current flows between the drain and source terminals. When a positive Vgs is applied, it creates an...

mosfet: depletion mode

Depletion MOSFET Channel Control

Depletion-mode MOSFETs are normally on transistors with a built-in channel that conducts current without gate bias. They work differently from enhancement-mode MOSFETs, which need a positive gate-source voltage, or V gs, to turn on. In a depletion device, the channel is already open because the channel doping creates a low-resistance path between the drain and source. These devices behave much like junction field-effect transistors, or JFETs. The channel conductivity is set by the material...

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...

small-signal analysis of mosfet amplifiers

small-signal-analysis-of-mosfet-amplifiers

MOSFET Amplifier Gain and Signal Distortion

MOSFET amplifiers can be treated as linear circuits when they operate in the saturation region. The gate-to-source voltage, or VGS, is the sum of a DC bias voltage and a small time-varying input signal. That bias sets the operating point and controls the drain current that flows from drain to source. When a small AC signal is added at the gate, the drain current has three parts. One part is the DC bias current, ID0, which is the quiescent current with no signal applied. A second part changes...

key terms and concepts

Explore how small-signal analysis links BJT transconductance, voltage gain, phase shift, and input resistance for amplifier circuits in grades 9–12.

BJT Amplifier Gain and Input Resistance | 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.

electrical engineering

BJT Amplifier Gain and Input Resistance

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