Power Amplifier and Differential Amplifier

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<p>AMPLIFIER TERMS Classes of Operation Class A operation the transistor operates in the active region at all times the collector current flows for 360 of the ac cycle the designer usually tries to locate the Q point somewhere near the middle of the load line. the signal can swing over the maximum possible range without saturating or cutting-off the transistor, which would distort the signal. AMPLIFIER TERMS Class B Operation the collector current flows for only half the cycle (180) a designer locates the Q point at cut-off only the positive half of ac base voltage can produce collector current reduces heat in power transistors AMPLIFIER TERMS Class C Operation the collector current flows for less than 180 of the cycle only part of the positive half-cycle of ac base voltage produces collector current brief pulses of collector current AMPLIFIER TERMS Types of Coupling Capacitive Coupling the coupling capacitor transmits the amplified ac voltage to the next stage ac coupling blocks the dc voltage AMPLIFIER TERMS Transformer Coupling the ac voltage is coupled through a transformer to the next stage ac coupling blocks the dc voltage AMPLIFIER TERMS Direct Coupling there is a direct connection between the collector of the first transistor and the base of the second transistor both the dc and ac voltages are coupled there is no lower frequency limit dc amplifier AMPLIFIER TERMS Ranges of Frequency Audio Amplifier an amplifier that operates in the range of 20Hz to 20kHz Radio-Frequency (RF) Amplifier one that amplifies frequencies above 20 kHz, usually much higher Narrowband Amplifiers works over a small frequency range Tunes RF Amplifiers their ac load is a high-Q resonant tank tuned to a radio station or television channel Wideband Amplifier operates over a large frequency range Untuned their ac load is resistive AMPLIFIER TERMS Tuned RF Amplifiers AMPLIFIER TERMS Signal Levels Small-signal Operation the peak-to-peak swing in collector current is less than 10% of quiescent collector current Large-signal Operation a peak-to-peak signal uses all or most of the load line Stereo System : the small signal from a radio tuner, tape player, or compact disc player is used as the input to a preamp, an amplifier that produces a larger output suitable for driving tone and volume controls. The signal is then used as the input to a power amplifier, which produces output power ranging from a few hundred milliwatts up to hundreds of watts TWO LOAD LINES DC Load Line One way to move the Q point is by varying the value of R2 large values of R2 : IC(sat) = VCC / (RC + RE) very small values of R2 : VCE(cutoff) = VCC TWO LOAD LINES AC Load Line RE has no effect on the ac operation the ac collector resistance is less than the dc collector resistance when an ac signal comes in, the instantaneous operating point moves along the ac load line the peak-to-peak sinusoidal current and voltage are determined by the ac load line MPP &lt; VCC where: MPP maximum peak-to-peak output voltage TWO LOAD LINES Equation of the ac load line: IC = ICQ + VCEQ/rc + VCE/rc When the transistor goes into saturation: ic(sat) = ICQ + VCEQ/rc When the transistor goes into cut-off: vce(cutoff) = VCEQ + ICQ rc where: ic(sat) = ac saturation current ICQ = dc collector current VCEQ = dc collector-emitter voltage rc = ac resistance seen by the collector vce(cutoff) = ac cut-off voltage TWO LOAD LINES Clipping of Large Signals when the Q point is at the center of the dc load line, the ac signal cannot use all of the ac load line without clipping if the ac signal increases, a cut-off clipping will result TWO LOAD LINES If the Q point is moved higher, a large signal will drive the transistor into saturation a saturation clipping will occur TWO LOAD LINES A well-designed large-signal amplifier has a Q point at the middle of the ac load line results in a maximum peak-to-peak unclipped output ac output compliance TWO LOAD LINES Maximum Output Q point below the center of the ac load line: maximum peak (MP) = ICQ rc Q point above the center of the ac load line: maximum peak (MP) = VCEQ Maximum Peak-to Peak Output MPP = 2MP TWO LOAD LINES When the Q point is at the center of the ac load line: ICQ re = VCEQ The circuits emitter resistance can be adjusted to find the optimum Q point: RE = (RC + rc) / (VCC/VE) -1 CLASS A OPERATION Power Gain equals the ac output power divided by the ac input power Ap = pout / pin CLASS A OPERATION Output Power in rms volts: pout = vrms2 / RL in peak-to peak volts: pout = vout2 / 8RL maximum output power occurs when the amplifier is producing the maximum peak-to-peak output voltage vout = MPP2 / 8RL CLASS A OPERATION Transistor Power Dissipation Quiescent Power Dissipation : PDQ = VCEQ ICQ When signal is present: the power dissipation of a transistor decreases worst case: quiescent power dissipation power rating must be greater than PDQ CLASS A OPERATION Current Drain dc source has to supply a dc current Idc to the amplifier dc current is called current drain Idc has two components: the biasing current through the voltage divider collector current through the transistor CLASS A OPERATION Efficiency dc power supplied to an amplifier by the source: Pdc = VCCIdc efficiency used to compare the design of power amplifiers q = pout / Pdc x 100% ac output power divided by the dc input power CLASS A OPERATION Efficiency a way to compare two different designs because it indicates how well an amplifier converts the dc input power to ac output power between 0 and 100 percent the higher the efficiency, the better the amplifier is at converting dc power to ac power important in battery-operated equipment CLASS A OPERATION Efficiency in Class A Amplifier the maximum efficiency of a class A amplifier with a dc collector resistance and separate load resistance is 25% in some applications, the low frequency of class A is acceptable CLASS A OPERATION Example: If the peak-to-peak output voltage is 18V and the input impedance of the base is 100O, what is the power gain? What is the transistor power dissipation and efficiency? CLASS A OPERATION Class A Power Amplifier class A power amplifier driving a loudspeaker uses voltage-divider bias the ac input signal is transformer-coupled to the base produces voltage and power gain to drive the loudspeaker through the output transformer CLASS A OPERATION Class A Power Amplifier the load resistance is also the ac collector resistance the efficiency of this class A amplifier is higher Impedance-reflecting ability of a transformer: ( Np/Ns)2 the maximum efficiency increases to 50% CLASS A OPERATION Emitter-Follower Power Amplifier locate the Q point at the center of the ac load line to get maximum peak-to-peak output CLASS A OPERATION large values of R2 saturate the transistor IC(sat) = VCC / RE small values of R2 drive the transistor into cut-off VCE(cutoff) = VCC CLASS A OPERATION ac load line end points: ic(sat) = ICQ + VCE/re VCE(cutoff) = VCE + ICQre MPP &lt; VCC CLASS A OPERATION Q point is below the center of the ac load line: maximum peak (MP) = ICQre Q point is above the center of the load line: maximum peak (MP) = VCEQ CLASS B OPERATION Push-Pull Circuit clips off half a cycle use two transistors in a push-pull arrangement push-pull one transistor conducts for half cycle while the other is off and vice versa CLASS B OPERATION Advantages and Disadvantages there is no current drain when the signal is zero the maximum efficiency of a class-B push-pull amplifier is 78.5% the uses of transformer CLASS B OPERATION Class B Push-Pull Emitter Follower an npn emitter follower and a pnp emitter follower connected in push-pull arrangement CLASS B OPERATION DC Equivalent Circuit select biasing resistors to set the Q-point at cut-off this biases the emitter diode of each transistor between 0.6 and 0.7 ICQ = 0 VCEQ = VCC / 2 CLASS B OPERATION DC Load Line the dc saturation current is infinite dc load line is vertical difficult thing: setting up a stable Q point at cut-off CLASS B OPERATION AC Load Line operating point moves up along the ac load line voltage swing of the conducting transistor can go all the way from cut-off to saturation MPP = VCC CLASS B OPERATION AC Analysis almost identical to a Class-A emitter follower AV ~ 1 zin(base) = |RL CLASS B OPERATION Crossover Distortion distorted output signal crossover distortion the clipping occurs between the time one transistor cuts-off and the other one comes on there is a need to apply a slight forward bias to each emitter diode ICQ from 1 to 5% of IC(sat) CLASS B OPERATION Class AB a conduction angle between 180 and 360 CLASS B OPERATION Power Formulas CLASS B OPERATION Transistor Power Dissipation ideally: power dissipation is zero when there is no input signal input signal: PD(max) = MPP2 / 40 RL CLASS B OPERATION Example: The adjustable resistor sets both emitter diodes on the verge of conduction. What is the maximum transistor power dissipation? The maximum output power? If the adjustable resistance is 15O, what is the efficiency? CLASS B OPERATION Biasing Class B/AB Amplifiers Voltage-Divider Bias thermal runaway then the temperature increases, the collector current increases, the junction temperature increases even more, reducing the correct VBE CLASS B OPERATION Diode Bias compensating diodes produces the bias voltage for the emitter diodes Ibias = (VCC 2VBE) / 2R ICQ has the same value as Ibias CLASS C OPERATION Class C Operation the collector current flows for less than half a cycle parallel resonant circuit: can filter the pulses of collector current and produce a pure sine wave of output voltage tuned RF amplifiers maximum efficiency can be 100% CLASS C OPERATION Tuned RF Amplifier resonant frequency: fr = 1 / 2t\ LC always intended to amplify a narrow band of frequencies ideal for amplifying radio and television signals CLASS C OPERATION Load Lines CLASS C OPERATION DC Clamping of Input Signal input signal: drives the emitter diode amplified current pulses: drives the resonant tank circuit the input capacitor is part of a negative dc clamper the signal appearing across the emitter diode is negatively clamped CLASS C OPERATION Filtering Harmonics harmonics - multiples of the input frequency; equivalent to a group of sine waves with frequencies of f, 2f, 3f,..., nf resonant tank circuit has a high impedance only at the fundamental frequency f CLASS C OPERATION Class C Formulas tuned class C amplifier a narrowband amplifier Bandwidth : BW = f2 f1 BW = fr / Q a large sinusoidal voltage at resonance with a rapid drop-off above and below resonance CLASS C OPERATION Current Dip at Resonance tune a resonant tank: look for a decrease in the dc current supplied to the circuit measure the current Idc from the power supply while tuning the circuit at resonant frequency: the ammeter reading will dip to a minimum value the tank has a maximum impedance at this point CLASS C OPERATION AC Collector Resistance QL = XL / RS RP = QLXL at resonance: XL cancels XC rc = RP//RL Q = rc / XL CLASS C OPERATION Duty Cycle D = W / T the smaller the duty cycle, the narrower the pulses compared to the period typical class C amplifier has a small duty cycle the efficiency of a class C amplifier increases as the duty cycle decreases CLASS C OPERATION Conduction Angle equivalent way to state the duty cycle D = | / 360 CLASS C OPERATION Transistor Power Dissipation maximum output: MPP = 2 VCC VCEQ &gt; 2VCC conduction angle is much less than 180 the collector current reaches a maximum value of IC(sat) peak current rating &gt; IC(sat) power dissipation depends on the conduction angle PD = MPP2/40rc CLASS C OPERATION Stage Efficiency for a conduction angle of 180, the average or dc collector current is IC(sat) /t optimum stage efficiency varies with the conduction angle CLASS C OPERATION Example: If QL is 100, what is the bandwidth of the amplifier? DIFFERENTIAL AMPLIFIER Differential Amplifier two CE stages in parallel with a common emitter resistor two input voltages: v1 (non-inverting) and v2 (inverting) two collector voltages no lower cut-off frequency differential output: vout = vc2- vc1 differential input: vout = AV(v1-v2) DIFFERENTIAL AMPLIFIER Single-Ended Output floating load neither end of the load can be grounded single-ended one end is grounded output voltage: vout = AV(v1-v2) the voltage gain is half as much as with a differential output DIFFERENTIAL AMPLIFIER Non-inverting Input and a Differential Output vout = AV(V1) Non-inverting Input and a Single-ended Output vout = AV(V1) but AV will be half as much DIFFERENTIAL AMPLIFIER Inverting Input and Differential Output vout = - AV (v2) Inverting Input and Single-ended Output vout = -AV (v2) but voltage gain will be half as much DIFFERENTIAL AMPLIFIER DC Analysis of a Differential Amplifier tail current: IT = VEE / RE emitter current of each transistor: IE = IT / 2 dc voltage on either collector: VC = VCC - ICRC DIFFERENTIAL AMPLIFIER DC Analysis Second Approximation IT = (VEE VBE) / RE Effect of Base Resistors: IT = (VEE-VBE)/ RE + RB/2|dc) DIFFERENTIAL AMPLIFIER Example: Calculate the currents and voltages using ideal and second approximations. AC ANALYSIS OF A DIFFERENTIAL AMPLIFIER Non-inverting Input and Single-ended Output the two halves of a differential amplifier respond in a complementary manner to the non-inverting input Q1 acts like an emitter follower that produces an ac voltage across the emitter resistor the amplified output sine wave is in phase with the non-inverting input AC ANALYSIS OF A DIFFERENTIAL AMPLIFIER Single-Ended Output Gain each transistor has an re the biasing resistor RE is in parallel with the re of the right transistor RE is much greater than re AC ANALYSIS OF A DIFFERENTIAL AMPLIFIER Simplified Equivalent Circuit input voltage vi across the first re is in series with the second re ac voltage across the tail resistor is half of the input voltage Single-ended output: AV=RC/2re AC ANALYSIS OF A DIFFERENTIAL AMPLIFIER Differential Output Gain the output voltage is twice as much since there are two collector resistors Differential Output: AV = RC / re AC ANALYSIS OF A DIFFERENTIAL AMPLIFIER Inverting-Input Configurations the inverting input v2 produces an amplified and inverted ac voltage at the final output AC ANALYSIS OF A DIFFERENTIAL AMPLIFIER Differential-Input Configurations both inputs are active at the same ti...</p>


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