The main contents of the chapter consist of the following: Introduction, the BJT internal capacitances, high-frequency BJT model, the high-frequency hybrid.
COMSATS Institute of Information Technology Virtual campus Islamabad Dr. Nasim Zafar Electronics 1 EEE 231 Fall Semester – 2012 The BJT Internal Capacitance and High Frequency Model Nasim Zafar Lecture No. 26 Reference: The BJT Internal Capacitance and HighFrequency Model Chapter5.8 Microelectronic Circuits Adel S. Sedra and Kenneth C. Smith. Nasim Zafar The BJT Internal Capacitances Nasim Zafar Introduction Ø Ø So far, we have assumed transistor action to be instantaneous The models we have developed, do not include any elements like capacitors or inductors, that would cause time or frequency dependence. Nasim Zafar Introduction Ø Ø Ø Actual transistors, however, exhibit charge storage phenomena that limit the speed and frequency of their operation. In this lecture, we study the chargestorage effects that take place in the BJT and take them into account by adding capacitances to the hybridπ model Nasim Zafar BJT: Small Signal Model We now again, define some quantities: IC qI C gm = = VBE kT −1 � I B � kT kT = rπ = � �= qI C � VBE � qI B Nasim Zafar IC IB BJT: Small Signal Model So β0 rπ = gm The output resistance is: −1 � I C � VA r0 = � �= � VCE � I C HighFrequency BJT Model C Cb C je Nasim Zafar HighFrequency BJT Model Ø The BJT inherently has junction capacitances which affect its performance at high frequencies. Cb represents the base charge Collector Junction: depletion capacitance, Cμ Emitter Junction: depletion capacitance, Cje, and also diffusion capacitance, Cb b je C C Nasim Zafar C 10 The Cutoff and UnityGain Frequency (cont’d) Ø Ø Slide 28 shows a Bode plot for hfe . From the –6dB/octave slope it follows that the frequency at which hfe drops to unity, which is called the unitygain bandwidth ωT, is given by: ωT = β 0ωβ Nasim Zafar 27 The Cutoff and UnityGain Frequency (cont’d) (C T Nasim Zafar C )r gm C C 28 The Cutoff and UnityGain Frequency (cont’d) gm 1= 2π fT (Cπ + Cµ ) gm 2π fT = (Cπ + Cµ ) kT = τt + (CdBE + CdBC ) 2π fT qI C Nasim Zafar 29 The Cutoff and UnityGain Frequency (cont’d) v Typically, fT is in the range of : v 100 MHz to tens of GHz. Nasim Zafar 30 Maximum Oscillation Frequency (fmax) Ø Ø One final important figure of merit is the MAXIMUM OSCILLATION FREQUENCY (fmax) Frequency at which unilateral power gain becomes 1 1/ f max � fT � =� � �8π rbCdBC � Nasim Zafar 31 Frequency Response of the CE Amplifier Nasim Zafar 32 High Frequency “RollOff” in Av Ø Typically, an amplifier is designed to work over a limited range of frequencies – At “high frequencies”, the gain of an amplifier decreases Nasim Zafar 33 Frequency Response of a CE Amplifier v The voltage gain of an amplifier is typically flat over the mid frequency range, but drops drastically for low or high frequencies. A typical frequency response is shown below LM(A vi ) = 20log(v o /vi ) [in dB] LM Response for a General Amplifier 20log(A vi (mid)) BW 3dB fLOW fHIGH Nasim Zafar f 34 Frequency Response of a CE Amplifier Av RollOff due to CL v High Frequency Band: A capacitive load (CL) causes the gain to decrease at high frequencies – The impedance of CL decreases at high frequencies, so that it shunts some of the output current to ground Av Nasim Zafar gm RC || j CL 35 Frequency Response of a CE Amplifier (contd.) v Low Frequency Band: At low frequencies, the capacitor is effectively an open circuit, and Av vs. ω is flat. At high frequencies, the impedance of the capacitor decreases and hence the gain decreases. The “breakpoint” frequency is 1/(RCCL) Av g m RC C R C L The CommonEmitter Amplifier Nasim Zafar 37 Frequency Response of a CE Amplifier Nasim Zafar 38 Frequency Response of a CE Amplifier v Ø v Ø Low frequency Band: For a CommonEmitter BJT: gain falls off due to the effects of capacitors CC1, CC2, and CE. Highfrequency Band: is due to device capacitances Cπ and Cμ (combined to form Ctotal) Nasim Zafar 39 Frequency Response of a CE Amplifier (contd.) Ø Ø Each capacitor forms a break point (simple pole or zero) with a break frequency of the form f=1/(2πREqC), where REq is the resistance seen by the capacitor. CE usually yields the highest lowfrequency break which establishes fLow Nasim Zafar 40 Amplifier Figure of Merit (FOM) v The gainbandwidth product is commonly used to benchmark amplifiers. – v We wish to maximize both the gain and the bandwidth Power consumption is also an important attribute – We wish to minimize the power consumption Gain Bandwidth Power Consumption g m RC RC C L I CVCC VT VCC C L Operation at low T, low VCC, and with small CL superior FOM Nasim Zafar 41 ... High Frequency Model Nasim Zafar Lecture No. 26 Reference: The BJT Internal Capacitance and HighFrequency Model Chapter5.8 Microelectronic Circuits Adel S. Sedra and Kenneth C. Smith. Nasim Zafar. .. (typically, 0.75 V), and m is its grading coefficient (typically, 0.2–0.5) Nasim Zafar 16 Junction Capacitances v Collector Junction: depletion capacitance, Cμ v Emitter Junction: depletion capacitance, Cπ Cµ = CdBC and C Cb C je Nasim Zafar 17 Nasim Zafar. .. and take them into account by adding capacitances to the hybridπ model Nasim Zafar BJT: Small Signal Model We now again, define some quantities: IC qI C gm = = VBE kT −1 � I B � kT kT = rπ = � �= qI C � VBE � qI B Nasim Zafar IC IB BJT: Small Signal Model