Transistors Amplifiers
P517/617 Lec6, P1Transistors AmplifiersCommon Emitter Amplifier ("Simplified")l What's common (ground) in a common emitter amp?The emitter! The emitter is connected (tied) to ground usually by a capacitor. To an ACsignal this looks like the emitter is connected to ground. What use is a Common Emitter Amp?Amplifies the input voltage (the voltage at the base of the transistor).The output voltage has the opposite polarity as the input voltage.We want to calculate the following for the common emitter amp:Voltage Gain Vout/Vi nInput ImpedanceOutput Impedance DC Voltage Gain:The voltage gain we are about to derive is for small signals only. A small signal is definedhere to be in the range of a few mV.As in all of what follows we assume that the transistor is biased on at its DC operating point.V out V cc - IC RCSince Vcc is fixed (its a DC power supply) we have for a change in output voltage VoutD V out -D IC RCIn the above D stands for a small change in either the voltage or current. The input voltage isrelated the emitter voltage by a diode drop:V i n VB VE 0.6 VDVi n DVEWe want to relate the emitter voltage to the emitter current (IE):V E IE RED V E D IE RE
P517/617 Lec6, P2We can relate the emitter and collector currents by remembering that for a transistor:IE ª ICD IE ª D ICFor now we assume that the currents are equal and rewrite the above equation for the emitterin terms of the collector current.D V E D IE RE DIC REWe also have D V E D Vi n so we can write the above as:D V i n D IC RE (- DV out / RC )REFinally we can write the DC voltage gain (G) for a common emitter amp as:DV outRGain - CD Vi nRENote: the minus sign in the gain means that the output is the opposite polarity as the input(1800 out of phase).What happens if RE 0? Do we have infinite gain?(must consider the AC case where XCE R)NO, we get a new model for the transistor.Remember that the base-emitter junction is a diode. We can describe the behavior of thejunction using the Ebers-Moll equation:I Is [e qV/ kT - 1]with V VBE and kT/q 25 mV at 200 C.Neglecting the -1 term:kTV BE [ln I - ln Is ]qWe wish to find the dynamic resistance of the base-emitter junction,dVrBE BEdIkT qI 25 10 -3 / I(note : for 1 mA of current rBE 25 W)RCGain rBE RE XCEWe can now write the gain for the case RE 0 (neglecting XCE too):Gain -RC / rBE - RC (IC / 25) with IC measured in mA.
P517/617 Lec6, P3Note: Simpson (page 227) writes an equivalent formula for the gain using the transistor parameterb and a slightly different temperature, T 3000 K.In terms of the hybrid parameter model (we will see this model soon)rBE hie / hfeUsing rBE to design a circuit is a dangerous practice as it depends on temperature and variesfrom transistor to transistor (even if they are the same type of transistor). Input impedanceThe input impedance of the common emitter amp can be calculated from the followingequivalent circuit:VinR1R2R tin1111 Ri n R1 R2 RtinDVBD IBDVE D IE / bDI E RE D IE / bRtin ª b REFor AC case R1 and R2 are usually Rtin so the input impedance is given byRtin bRE b rBE 2500 W for 1 mA of collector current and b 100. Output impedanceThis is harder to calculate than the input impedance and only a hand waving argument for itsvalue will be given here. The output impedance of the amp is the parallel impedance of RC and theoutput impedance of the transistor looking into the collector junction. The collector junction isreversed biased and hence looks like a huge resistor compared to RC.Thus the output impedance is simply RC assuming that the load impedance (the thing the ampis hooked up to) is less than RC.
P517/617 Lec6, P4The Common Collector Amplifier:Sometimes this amp is called an emitter follower.What's common (ground) in a common collector amp?The collector! The collector is connected (tied) to a DC power supply. To an AC signal thislooks like the collector is connected to ground.VccRsCinVinR1R2Q1C0VoutREWe want to calculate: voltage and current gain, and input and output impedance. Voltage Gain: The input is the base and the output is taken at the emitterV E V B - 0.6 VD V E D VBD V out D Vi nThus the amp has unity gain! Current Gain: As always we can use Kirchhoff's current rule.IE IB IC IB (b 1)D IE b 1D IBD Iout b 1D Ii nSince a typical value for b is 100, there is lots of current gain.
P517/617 Lec6, P5 Input impedance:By definition the input impedance isDV i nRi n D Ii nDV BD IBDVE DI E / (b 1)D IE RE DI E / (b 1) Ri n (b 1)RESince RE is usually a few kW and b is typically 100, the input impedance of the commoncollector amp is large. Output impedance: This is trickier to calculate than the input impedance.In the figure below we are looking into the amp:RsR inVinRi n is the input impedance of the transistor and Vtin is the voltage drop across it.V RVtin in inRin RsªVinbREbRE RsIf we now look from the other (output) side of the amp with Rout the output impedance of thetransistor, the voltage drop at A is the same as the voltage at the base (VB) since a common collectoramp has unity gain. We can rewrite the equation into a voltage divider equation to find Rout.Rout VAREVinVA Vin RERE RoutVinbREVin RER or Rout sbRE Rs RE Rs / bbThus Rout is small since b is typically 100. Vtin
P517/617 Lec6, P6 What good is the common collector amp?Example: In stereo systems very often loud speakers have 8 W input impedance. Assume that youwant to drive the speakers with a 5 Volt 92 W voltage source. Lets look at 2 ways of driving thespeakers and the power each method delivers to the speaker.a) Hook the speakers directly to the voltage source:92 WVin8 W speaker(5 V rms)The voltage delivered to the speaker is (8/100)Vi n. The power delivered is:P V 2 / R (5 8 / 100)2 / 8 0.02 Watts (not much power!)b) Use a common collector (emitter follower):VccVin(5 V rms)92 WCin 50 K R 15KR2Q1C0Vout8Wspeakers2 K REAn AC signal at the input sees bRsp b8 W 800 W .Vin92 W 5 K 50 Kb2Kb8WFrom the speakers point of view the amp impedance looks like 92 W/b or about 1 W. Thepower delivered to the speaker can now be calculated:Vsp (b 8 WV i n) / (b8 W 92 W) 0.9Vi n (Volts )2Psp Vsp/ Rsp (0.9 5)2 / 8 2.5 Watts (rms)Thus there is more than a factor of a hundred times more power delivered to the speakerwith an emitter follower amp.Emitter Followers (common collectors) are used to match high impedances to low impedances.
The output impedance of the amp is the parallel impedance of RC and the output impedance of the transistor looking into the collector junction. The collector junction is reversed biased and hence looks like a huge resistor compared to RC. Thus the output impedance is simply RC assuming th
RF/IF Differential Amplifiers 5 Low Noise Amplifiers 7 Low Phase Noise Amplifiers 10 Gain Blocks 11 Driver Amplifiers 13 Wideband Distributed Amplifiers 13 Power Amplifiers 15 GaN Power Amplifiers 18 Variable Gain Amplifiers 19 Analog Controlled VGAs 19 Digitally Controlled VGAs 20 Baseband Programmable VGA Filters 21 Attenuators 22
RF/IF Differential Amplifiers 9 Low Noise Amplifiers 9 Gain Blocks and Driver Amplifiers 10 Wideband Distributed Amplifiers 11 Linear and Power Amplifiers 12 GaN Power Amplifiers 13 Active Bias Controllers 13. Variable Gain Amplifiers 14. Analog Controlled VGAs 14 Digitally Controlled VGAs 14 Baseband Programmable VGA Filters 14. Attenuators 15
Semiconductor optical amplifiers (SOAs) Fiber Raman and Brillouin amplifiers Rare earth doped fiber amplifiers (erbium – EDFA 1500 nm, praseodymium – PDFA 1300 nm) The most practical optical amplifiers to date include the SOA and EDFA types. New pumping methods and materials are also improving the performance of Raman amplifiers. 3
gate JL transistors, multi-gate JL transistors, silicon on insulator (SOI) JL transistors, and gate all around JL transistors. . For the SHE in FinFETs, various methods and the structures of transistors able to reduce this effect were considered in many works [11-15]. In [11], the charge plasma (CP) based JL MOSFET on a selective buried .
51.3 Types of Semiconductor Optical Amplifiers SOAs can be classified as either subthreshold or gain clamped. Subthreshold amplifiers are lasers operated below threshold, and gain-clamped amplifiers are lasers operated above threshold but used as amplifiers. Subthreshold SOAs can be further classified according to whether optical feedback .
Optical Amplifiers vs Regenerators (1 of 2) Transparent: Regenerators specific to bit rate and modulation format used; O-Amps are insensitive Easily upgraded: A system with optical amplifiers can be more easily upgraded to higher bit rate without replacing the amplifiers Optical amplifiers
sistor amplifiers. Thirty years ago, transistor amplifiers were all the rage and valve amplifiers had become passe. Anybody who was anybody was discarding their large, hot and fragile valve amplifiers for small, cool, and — generally far more expensive — transistor amplifiers. Mullard and Philips had released their OC16's, which
Physical limits of silicon transistors and circuits 2703 1. Introduction Readers may be surprised to find a paper on transistors in a physics journal. However, the transistor was invented by physicists and the subject of transistors is permeated with physics. The early studies of transistor
