11Chapter Goals• Bipolar switching circuits• Emitter-coupled logic (ECL)• Behavior of the bipolar transistor as a saturated switch• Transistor-transistor logic (TTL)• Operation of the transistor in the inverse-active region• Schottky clamping techniques for preventing saturation• BiCMOS logic circuits2Introduction of BiCMOS Digital Circuits• BiCMOS is a VLSI technology that combines bipolar and CMOS devices into single integrated circuit.• By combining the two technologies BiCMOS offers the following advantage:•i) Low power dissipation comparable to CMOS •ii) Improved speed comparable to TTL or ECL technology • iii) Large current driving capability comparable to TTL or ECL•iv) large noise margin similar to TTL technologyDisadvantage:i) Highest costii) Large fabrication cycle time up to thirty mask steps are common compared with ten to twenty for bipolar or CMOS.13BiCMOS Logic• BiCMOS is a complex processing technology that provides both NMOS and PMOS devices, as well as npn and pnp bipolar transistors• High input impedance logic gates (that require little drive current) are provided by the MOSFETs, and high current drive can be provided from the BJTs due to their high current gain and transconductance4BiCMOS Inverter (when input is low)• Case I: When VI=logic 0The MNand Q2 are off while Mp conducts, which forces Q1 on. The Q1 then provide a large output current to charge the load capacitance. The result is very short low-to-high propagation delay time. Q1is essentially acts as a pull up transistor and V0(max)=VDD-VBE1(on),because Q1turn off when Vo reaches to this value, a disadvantage.225BiCMOS Inverter (when input is high)• Case II: when VI = logic 1Mp goes off, MNand Q2turn on. The transistor Q2provide large output current that quickly discharge the load capacitor, which result a short high to low propagation delay time. The output voltage continue to decrease untilVO(min)=VBE2(on)Q2, because Q2turn off at this value. Thus low voltage is greater than 0, a disadvantage.36Improved version of BiCMOS inverter•One serious disadvantage of the previous circuit we discussed is that the absence of circuit path through which base charge can be removed from the npn transistors (Q1and Q2), when they are turn off.• The solution of this problem is addition of pull down resistors (R1 and R2) as shown in the figure.•Resistor R2provides an additional benefit:When VIhigh and after Q2cuts off, Vo continue to fall below VBE2 (on) because a small current through MN and R2continue to flow , which pulled the output to ground potential.• Similarly, when output goes high, a very small current through Mp and R1 pulled up the output to VDD.47BiCMOS Logic Gate• More complex logic gates can also be implemented using BiCMOSdesignTwo-input BiCMOSNOR gate8Review39Ideal Operational Amplifier• The ideal op amp is a special case of an ideal differential amplifier with infinite gain, infinite Ridand zero Ro.– If A is infinite, vidis zero for any finite output voltage.– Infinite input resistance Ridforces input currents i+and i-to be zero.• The ideal op amp has the following assumptions:– Infinite common-mode rejection, power supply rejection, open-loop bandwidth, output voltage range, output current capability and slew rate– Zero output resistance, input-bias currents and offset current, input-offset voltage.vid=vOA and limA→∞ vid=01011i1 = i2V0=V1-i2R2=0 - (VI/R1)R212413⇒1415If the op amp open loop gain is finite (non ideal op amp)16517 1819 20If R2=0 or R1=∞621Demonstration of the Voltage Follower as a perfect bufferBy voltage divider rule,⇒A severe loading effect or attenuation.RS<< input impedance of op amp⇒VO≅ VI⇒ Loading effect is eliminatedi=022There are number of ways to solve this problem; perhapsthe easiest is using the principle of superposition and virtual short concept.To apply superposition we first reduce VI2=023What is the condition under which this circuit will act as a difference amplifier?Let VI1=VI2⇒VO=0⇒Which is clearly that of a difference amplifierwith a gain of R2/R124725 26Common modegain andcommonmode rejectionratio27The current in the resistor R1 is (by superposition rule)Substituting above equationsClearly the input impedance is very high (why?) and the gain Value can be varied by varying the R1.28i1i1i1=VI/R1, this current also flow through the capacitor, causing charge to accumulate on VC.At time t, the charge Q at the capacitance equal to dttit)(01∫Because i=Q/t and the voltage acrossthe capacitor is dttict)(101∫Because Q=CV. If initial voltage (t=0) on C is denoted VCdtticVtVtCC)(1)(01∫+=Now the output voltage VO=-VC(t)dttiCVVtCO)(101∫−−=dttVCRVVtICO)(10∫−−=In terms of the voltageThus the circuit provides an output voltage that is proportional to timeintegral of the input.829 30We know the current I is the rate of change of chargeiii=dQ/dt, also Q=CVdtdvCvCdtdiII 11)( ==Since A is a virtual groundAdRdtdvCiRvIO)(.22−=−=dttdvCRvIO)(12−=31Chapter FifteenApplications and Design of Integrated Circuits32Passive and active filters• Filters are building blocks of communication and instrumentations.• The oldest technology based on inductors and capacitors are called passive LC filters, which are incompatible with any of the modern techniques for assembling electronic systems.• Active-RC filters utilize op amp together with resistors and capacitors.• At present, the most viable approach for realizing fully integrated monolithic filters is the switch capacitance techniques.933One pole active filters• Usually, passive filters suffers from loading effects, substantially reducing the maximum gain from the unity.• Loading effect can be reduced by using active filters.34(cont.)35 361037 3839 401141 42• An electronic oscillator may be define in any one of the following four ways:i) It is a circuit which convert dc energy into ac energy at a very high frequencyii) It is an electronic source of alternating current or voltage having sine, square or sawtooth or pulse shapesiii) It is circuit which generates an ac output signals without requiring any externally applied input signal.iv) It is an unstable amplifier.43Classification of Oscillators• Electronic oscillators may be broadly divided into following two groups:i) sinusoidal (or harmonic) oscillators-which produce an output having sine wave formii) non- sinusoidal (or relaxation) oscillators-they