Transistor Amplifier Theory

Transistor Amplifier Theory

• Bias establishes the DC operating point for the linear operation of an amplifier.
• Without correct DC voltages on the input and output, an amplifier can go into saturation or cut-off when an input signal is applied.
• Diagrams illustrate the effects of proper and improper DC biasing of an inverting amplifier.
• In the diagrams, the output signal is an amplified replica of the input signal, but it is inverted (out of phase with the input).
• The output signal symmetrically swings above and below the DC bias level of the output, VDC(out).
• Improper biasing can cause distortion in the output signal.

Correct DC Bias

• Limiting of the positive portion of the output voltage occurs when the DC operating point (Q-point) is too close to cut-off.
• Limiting of the negative portion of the output voltage occurs when a DC operating point is too close to saturation.

Graphic Analysis

• A transistor is biased with variable voltages VCC and VBB to obtain certain values of IB, IC, IE, and VCE.
• Collector characteristic curves illustrate the effects of DC bias.

DC Load Line Analysis

• When Iß increases, Ic increases and VCE decreases. Conversely, when Iß decreases, Ic decreases and VCE increases.
• As VBB is adjusted, the DC operating point of the transistor moves along the DC load line.
• The DC load line connects each Q-point, the DC operating point specified by voltage and current values, referring to a quiescent state.
• Values of IB, Ic, and VCE can be determined at any point along the line.
• The DC load line intersects the VCE axis at 10 V, where VCE = VCC, which represents the transistor cut-off point.
• Ideally, Iß and Ic are zero at cut-off, but there is a small leakage current, ICBO, which can typically be neglected.
• The DC load line intersects the Ic axis at 45.5 mA, representing the transistor saturation point.
• At saturation, Ic is maximum, VCE = 0 V, and Ic = Vcc/Rc.
• A small voltage (VCE(sat)) exists across the transistor, making IC(sat) slightly less than 45.5 mA.

Transistor Linear Operating Region

• The region along the load line between saturation and cut-off is known as the linear region, allowing linear reproduction of the input.
• In linear operation, a sinusoidal voltage Vin superimposed on VBB causes sinusoidal variation in base current (±100 µA around the Q-point value of 300 μA).
• As a result, collector current varies (±10 mA around the Q-point value of 30 mA), and collector-to-emitter voltage varies (±2.2 V around the Q-point value of 3.4 V).
• Point A corresponds to the positive peak, Point B to the negative peak, and Point Q to the zero value of the sine wave on the load line.
• VCEQ, ICQ, and IBQ are DC Q-point values without an input sinusoidal voltage.

Waveform Distortion

• Certain input signal conditions can position the Q-point on the load line to cause one peak of the VCE waveform to be limited (clipped).
• This is seen in the waveform distortion diagram, where the input signal is too large for the Q-point location.

Voltage-Divider Bias

• It is more practical to use Vcc as a single bias source rather than a separate DC source to bias the base-emitter junction.
• A resistive voltage-divider (R₁ and R2) develops a DC bias voltage at the base of the transistor. Vcc is the DC collector supply voltage.
• There are two current paths from point A to ground: through R2 and through the base-emitter junction and RE.
• Fixed-bias resistors, R₁ and R2, maintain a constant base bias while emitter bias changes with emitter conduction, which improves thermal stability.
• Resistor RE, in series with the emitter, is the emitter bias. If IE increases due to temperature, the voltage drop across Re also increases, reducing VCE, thus counteracting the variation in IE and regulating against thermal changes.

Base Bias

• Base bias consists of a resistor (RB) connected between the collector supply voltage and the base.
• This arrangement is thermally unstable. As temperature rises, Ic increases.
• Increased current causes the DC operating point (Q-point) to move from the desired position, affecting amplifier gain and potentially causing distortion.

Collector-Feedback Bias

• ẞDC varies directly with temperature, and VBE varies inversely with it.
• In a collector-feedback circuit, rising temperature causes ẞDc to increase and VBe to decrease.
• BDc increases, increasing Ic, and VBE decreases, increasing Iß, which also increases Ic
• As Ic rises, the voltage drop across RC increases, reducing collector voltage across RB, which reduces Iß which offsets the increase in Ic.
• It maintains a relatively stable Q-point. Reverse action occurs when temperature decreases.

The Linear Amplifier

• A voltage-divider biased transistor has a sinusoidal AC source capacitively coupled to the base through C₁ with a load capacitively coupled to the collector through C2.
• The capacitors block DC, preventing the source resistance (Rs) and load resistance(RL) from changing the DC bias voltages, acting as shorts to the signal voltage.
• The sinusoidal source voltage causes base voltage to vary sinusoidally around its DC bias level. The resulting base current variation produces a large variation in collector current due to the transistor's current gain.
• As the sinusoidal collector current increases, the collector voltage decreases, varying above and below its Q-point value in phase with the base current.

The Linear Amplifier: Circuit Voltage Relationships

• The sinusoidal collector-to-emitter voltage varies above and below its Q-point value at 180° out of phase with the base voltage.
• A transistor always produces a phase inversion between the base and collector voltage.

A Graphical Picture

• The transistor's operation can be graphically illustrated using collector-characteristic curves.
• Sinusoidal voltage at the base produces a base current that varies around the Q-point on the AC load line.
• Lines projected from the peaks of the base current to the IC and VCE axes indicate the variations of collector current and collector-to-emitter voltage.
• The AC load line differs from the DC load line as the effective AC collector resistance is R₁ in parallel with Rc, lower than DC collector resistance R alone.

Thermal Stabilisation

• Overcome the problem of larger high-powered transistor heat generation using devices with negative temperature coefficients, like a thermistor or diode within the transistor casing connected to to lower arm of base bias divider.
• The purpose is to counter the impact of increased temperature on base bias, reducing it through the use of these devices.
• An emitter resistor (R2) provides proper biasing and temperature stability in a common-emitter transistor amplifier.
• There will be a signal development at the emitter in phase with the input signal on the base caused by the changing current through the emitter resistor (R2) as the current through the transistor changes.

Emitter Signal as Negative Feedback

• Emitter signal is a form of negative feedback.
• Base-to-emitter bias controls current through the transistor. Applying simultaneous positive voltage to the base and emitter leads to no increase in current because base-to-emitter voltage difference stays consistent.
• This issue can remedied be coupling the signal to ground, also known as 'decoupling'.
• Placing the Decoupling capacitor (C3) (also called bypass capacitors) across the emitter (Q₁) to the ground (across the emitter resistor) easily passes the signal without affecting the resistor.
• Bypassing capacitor does not eliminate the negative feedback entirely therefore negative feedback improves the amplifier's stability and positive feedback adds to instability.
• It’s better to have three stage each with a gain of 10 than a single stage with a gain of 1000 since Higher gain stages are inherently unstable.

Common Emitter Amplifier

• The diagram illustrates a common emitter amplifier incorporating voltage divider bias, coupling capacitors (C₁ and C3) at the I/O, and a bypass capacitor (C2) that grounds the emitter.
• There's a blend of both DC and AC operations required in this kind of circuit.
• Capacitive coupling of the input signal (VIN).
• Amplifier output is 180⁰ out of phase due to CE (common-emitter).

Common Collector Amplifier

• Applied to input(Base) with a coupling capacitor, output(emitter) will roughly share its amplitude to its output.
• Key advantages include high input of resistance, current gains despite having ≈1 voltage gain.

Common Base Amplifier

• Offers high voltage gain in addition to a max of ≈1 current gain. Making it key for low resistance applications/outputs.

Amplifier Configuration Characteristics

• There are differences between each amplifier type in terms of gain, resistance, phase relation etc
• Common base amplifiers have a 0⁰ input/output phase relationship, high voltage gain, lower resistance while common emitters are its opposite having medium gain with resistance but and 180⁰ I/O phase relationship.
• Output resistance is low in common collector set ups where input resistance is high.

Amplifier Coupling: RC Coupling

• Resistance capacitance (RC) are multi-stage amplifier circuits coupled together in the form of Common Emitters design. Rc resistors work as a bias, also including a stabilization network/Emitter resistor(RE) where CE is a bypass capacitor only restricting the D/C voltage and letting alternating current drip across RL.
• Mid-frequencies are constant based on frequency response despite gain decreasing overtime as result of high CC reactance that decreases a signal.

LC Coupling Amplifier

• LC coupling is barely used. DC current coils(L1 and L2) are less resistant compared to RC, allowing for less power loss. But this makes LC coupling AC signal amplification sensitive to frequency increases/instability.

Transformer Coupled Amplifier

• Using Transformer coupling will allow for a circuit to create use of previously mentioned connections, now called TC amplifiers by having the transformer T1 primary replaces the CL with secondary winding connecting/providing input to the second stage for more amplifier stages.
• It employs a Biasing & Stabiliziation network formed by voltage divisor network alongside transistor coupling capacitors.

Operation and coupling

• These amplifiers amplify and use a signal through the collector, the transformer used has impedance control which can easily turn low stage resistance to high load for previous stages.
• This amplifier setup is used for power amplification as well.

Direct Coupled Amplifier

• It is important that DC and AC are coupled between stages in some amplifiers directly connecting the output(Collector) to input(base) of next stage without blocking DC. Useful for low-freq amplification.
• Direct coupling amplifiers also removes capacitors which might decrease frequency stability and lead to filter formations along capacitor/resistors that distort gain overtime.
• However, DC coupling is also causes temperature instability(due to fluctuation) creating error voltages between stages that get magnified at subsequent stages despite some feedback that assists stabilization.

Inverting / Non-Inverting Amplifiers

• Adding voltage values/signal strength can occur using inverting amplifiers depending if it has a separate circuit with inverted output by the formula:
• Adding Scaling can occur as well by increasing Input/output resistors such as using more input from a Tank A vs Tank B proportional fuel level tank readings.

Amplifier Classes of Operation

• Amplifier classes of operation are decided on the current flow the output circuit provides depending on the Amplifying device itself alongside the degree of bias applied.
• 3 Main Amplifier classes: A, B and C for purposes with varying characteristics, decided on circuit types over personal bias of a “best” setup.
• Having amplifier biased within the in the linear region for 360⁰ of the input cycle leads to class a amplifier being established, fully replicating input with potential in + out phase.

Class A Amplifier Operation

• Operates with 100/100 Signal output, having a consistent current.

Class B Amplifier Operation

• Works at 180⁰ to both output/cut which results in about 50% input signal.

Class B Amplifiers, Rectifiers

• May beg a question of why B amps are used over rectifiers despite only have input signal result. Key point to be stated is that the class B amplifier can only reproduce just the signal, whereas a rectifier cannot.
• Rectifiers have a max signal output that equals its output. B class amplifiers are also more efficient since its device can amplify despite its signal. A class C can also be employed if amps use less 50% input signal.

Push-Pull Amplifiers

• This two transistor configuration enables good reproduction of complete input 360⁰ signals with the use of common emitter followers.
• Amplifier uses transistors (NPN,PNP) in opposed alternations, requiring there to be no DC base bias for signal/voltage to drive Q1/Q2 properly especially at respective positive and negative half cycles.

Crossover Distortion

• Transistor conduction will require there to VBE >0. Otherwise there will time interval within potential alternations which cause distortion.

Push Pull Amplifier Biasing

• Eliminating crossover distortions require transistors in push-pull set up be just above their cut. Set ups can be designed with basic resistor/voltage dividers while other arrangements like transistors with proper diode characteristics are used for a steady bias.

Class C Operation

• amplifier where portions are barely within the output portion or signal, making them the most efficient amplifier where output signals bears less resemblance to their original input.
• Amplifier biased set such conduction happens at over 180° output signal where power dissipation is lower result of only conduction at small parts, mostly useful for tuned radio frequencies output amplifier.