Chapter 7Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Chapter 7Differential Amplifiers and Integrated Circuit (IC) AmplifiersDiscrete and Integrated CircuitsA discrete circuit is constructed of components that are manufactured separately. Later, these components are connected together, by conductors like wires, in a circuit board or a printed circuit board (PCB).On the other hand, in an integrated circuit, the components and their inter-connections are manufactured concurrently by a sequence of processing steps. The types of components that are available and their practical values depend heavily on the approach taken for implementation. (for example, the capacitors in discrete circuits can be in the range of 1pF to 1F, but only 1pF to 100pF in ICs. Also, inductors are almost impractical in ICs. But in ICs, matching of components is much easier)See table 7.1 in page 412 for detail.Applications of discrete circuits will persist especially for some special circuits that are to be mass produced, but today the bulk of electronic systems are based on ICs.Processing steps in manufacturing ICs incur cost and failures and are usually different for different technologies.BJT technology are used more for high-quality analog circuits, while MOS are more for general analog circuits and digital circuits. Today, semiconductor industry can manufacture both BJT and MOS on the same chip, called BiCMOS technology.Figure 7.1 The current mirror.DC biasing for Integrated CircuitsDifferent from biasing of discrete circuits, resistors and capacitors are “expensive” in terms of cost and chip area, are therefore avoided whenever possible.For amplifier circuits, the BJT transistors operate in active region.The following circuits show how matched transistors, when combined with a few resistors, can act as current sources that are useful in biasing IC amplifiers. Collector of Q1 is connected to its base. Thus , and Q1 is in the active region. If is larger than 0.2V, Q2 is also in the active region. See page 415-416, it can be shown that VVVBECE6.0112CEVRVVIIIBECCrefCC21Figure 7.1 The current mirror.DC biasing for Integrated Circuits IITo a first-order approximation, the base current of Q2 is independent of the output voltage , therefore the output characteristics is almost identical to one of the collector characteristic curves for Q2.An important specification of a current source is the range of output voltage for which the output current is approximately constant, which is called compliance range. Another important specification of a current source is its dynamic output resistance, which is the ratio of the incremental voltage divided by the incremental output current (ideally it should be infinite).In small-signal equivalent circuit, the current source is replaced by its dynamic resistance. 122)(CECOVIr2CEVFigure 7.2 Emitter follower with bias current source.Biasing an Emitter FollowerAn example of how the current mirror can help establish the bias point of an IC amplifier is shown below. The current source is formed by R, Q1 and Q2, while Q3 is an emitter follower amplifying the input signal and delivering it to the load.Often, we can simplify the circuit diagram as in Figure7.2(b).Note: (1) the amplifier is direct-coupled compared to AC-coupling in discrete amplifiers(2) Output voltage is -0.7V for input voltage of zero. In this case, the circuit displays a DC offset, which is not desirable. This problem can be solved or reduced by the circuit shown in the next slide.Figure 7.3 The offset voltage can be reduced by cascading a complementary (pnp) emitter follower.Biasing an Emitter Follower: reducing offsetA simple way to reduce offset for this follower is to cascade a second stage consisting of a pnp emitter follower as shown in the figure below. Note that in discrete circuits, offset is not an issue as a coupling capacitor is used.Figure 7.4 Doubling the junction area of a BJT is equivalent to connecting two of the original BJTs in parallel.Effects of transistor area on current mirrorDoubling the area of a transistor is the same as connecting two of the original transistors in parallel, as shown in the Figure 7.4.The output current of a current mirror for which the relative junction areas of the transistors are A1 and A2 is given byStudy example 7.1 in page 418. 121212AAIAAIIrefCCFigure 7.5 Current mirror for Examples 7.1. Figure 7.9 Collector characteristic of Q2, illustrating the Early voltage.Figure 7.7 Output characteristic for the current mirror of Figure 7.5.Figure 7.8 Dynamic output resistance of the current mirror of Figure 7.5.Figure 7.10 The Wilson current source, which has a high output resistance.The Wilson current sourceAn improved circuit, called Wilson current source, with higher output impedance that the previous current mirror is shown in the Figure. For the Wilson current source, the following holds:A1, A3 are the relative junction areas of the Q1 and Q3 respectively. (see page 421)refCBEBECCrefIAAIRVVVI13232Figure 7.11 The Widlar current source, which is useful for small currents.The Widlar current sourceWhen the desired current is small, the Widlar current source may be a better alternative, as shown in the Figure.For Widlar current source, the following holds (see page 422):See example 7.3 in page 423.1112122)ln(RVVIIIIIVRBECCrefCCCCTFigure 7.12 Typical biasing circuit for a bipolar IC.The combined current sourcesIn an Integrated Circuit amplifier, several current sources use the same reference current, as shown below. The current through R1 is the reference current for all four current sources. Q1, Q2 forms a current mirror, and Q1, Q3 forms a Widlar source. Notice the pnp current source by Q4, Q5 and Q6.Figure 7.15 NMOS current mirror.IC biasing with MOSFETThe BJT current sources have counterparts constructed with MOSFET.The shown MOSFET current mirror is very similar to the BJT current mirror.In typically cases, the MOSFET M1 operates in saturation region, as drain-to-gate voltage is zero. Assuming the transistors are identical and that the output voltage is large enough so that M2 is in saturation as well. The current