Integrated circuit (IC) technology permits the realization of large numbers of virtually identical transistors. Although the absolute parameter tolerances of these devices are relatively poor, de-vice characteristics can actually be matched to within less than 1 percent. The availability of large numbers of such closely matched devices has led to the development of special circuit techniques that depend on the similarity of device characteristics for proper operation. These matched cir-cuit design techniques are used throughout analog circuit design and produce high-performance circuits that require very few resistors.

• One of the most important of the IC techniques is the current mirror circuit, in which the output current replicates, or mirrors, the input current. Multiple copies of the replicated current can be generated, and the gain of the current mirror can be controlled by scaling the emitter areas of bipolar transistors or the W/L ratios of FETs. Errors in the mirror ratio of current mirrors are related directly to the finite output resistance and/or current gain of the transistors through the parameters λ, V_A, and β_F.
• In bipolar current mirrors, the finite current gain of the BJT causes an error in the mirror ratio, which the buffered current mirror circuit is designed to minimize. In both FETand BJT circuits, the ideal balance of the current mirror is disturbed by the mismatch indc voltages between the input and output sections of the mirror. The degree of mismatchis determined by the output resistance of the current sources.
• The figure of merit V_CS for the basic current mirror is approximately equal to V_A for the BJT or 1/λ for the MOS version. However, the value of V_CS can be improved by up to two orders of magnitude through the use of either the cascode or Wilson current sources.
• Current mirrors can also be used to generate currents that are independent of the power supply voltages. The V_BE -based reference and the Widlar reference produce currents that depend only on the logarithm of the supply voltage. By combining a Widlar source with current mirror, a reference is realized that exhibits first-order independence of the power supply voltages. The only variation is due to the finite output resistance of the current mirror and Widlar source used in the supply-independent cell. Even this variation can be significantly reduced through the use of cascode and Wilson current mirror circuits within the reference cell. Once generated, the stabilized currents of the reference cell can be replicated using standard current mirror techniques.
• The Widlar cell produces a PTAT voltage (proportional to absolute temperature) which is used as the basic sensing element in most electronic thermometers.
• The band gap reference combines a PTAT cell with a base-emitter voltage to produce a highly precise output voltage that is independent of temperature and supply voltage. The typical output voltage of the basic band gap cell is 1.20 V at room temperature and is approximately equal to the silicon band gap voltage. The 1.20-V output is easily scaled up to any desired reference voltage.
• An extremely important application of the current mirror is as a replacement for the load re- sistors in differential and operational amplifiers. This active-load circuit can substantially enhance the voltage gain capability of most amplifiers while maintaining the operating-point balance necessary for low offset voltage and good common-mode rejection. Ampli-fiers with active loads can achieve single-stage voltage gains that approach the amplification factor of the transistor. Analysis of the ac behavior of circuits employing current mirrors can often be simplified using a two-port model for the mirror.
• Active current mirror loads are used to enhance the performance of both bipolar and MOS operational amplifiers. The classic μA741 operational amplifier, introduced in the late 1960s, was the first highly robust design combining excellent overall amplifier performance with input-stage breakdown-voltage protection and short-circuit protection of the output stage. Active loads are used to achieve a voltage gain in excess of 100 dB in an amplifier with two stages of gain. This operational amplifier design immediately became the industry standard op amp and spawned many similar designs.
• Four-quadrant multiplication of analog signals can be accurately obtained using the Gilbert multiplier circuit.