Transistor biasing with current mirrors12/6/2023 Figure 2: Using different numbers of transistors on the input and output sides of a current mirror enables gains other than unity. All five transistors have the same V BE, and their collector currents, I C, will be equal. For instance, in a current mirror with two matched transistors on the input and three on the output, the mirror gain will be 3/2, or 1.5 (Figure 2). For example, if there are N-matched transistors on the input side and M-matched transistors on the output, the gain of the current mirror will be M/N. Arbitrary gains can be achieved by using various numbers of matched transistors on the input and output sides. But, even in those cases, it’s easier to use matched transistors and modify the current mirror circuit to produce gains other than one. When fabricating current mirrors in ICs, it’s possible to control transistor characteristics as needed to produce specific gains. There are instances where a gain other than one is desirable. When using matched transistors, the input current to output current ratio (gain) is one, and the currents are equal. Figure 1: A basic current mirror consists of two cascaded transistor stages acting as reversed and direct volage-to-current converters, respectively. If the transistors are matched and the mirror output voltage selected, so Q2 has a collector-base voltage of zero, then the V BE of Q1 causes the emitter current in Q2 to be the same as the emitter current in Q1. The voltage drop across Q1 is V BE, and Q1 sets V BE for Q2. The emitter of Q1 is connected to ground, and the collector-base voltage is zero. A simple current mirror consisting of two cascaded transistor stages acting as reversed and direct volage-to-current converters, respectively, implements this approach (Figure 1). It controls the output base-emitter voltage to pass the collector current. Connecting the base and collector results in negative feedback, and the transistor acts as an opposite voltage-to-current converter. In a basic BJT current mirror, the input transistor operates as an exponential voltage-to-current converter when a voltage applied to the BJT base-emitter junction acts as an input quantity, and the collector current is used as the output quantity. To overcome this problem, a single stable current source is implemented in the IC, and multiple copies of the stable source are generated using current mirror techniques. A single IC can include large numbers of amplifiers, and to bias, all the amplifiers to exactly the same level can be challenging. Compliance voltage range starts with the minimum voltage drop across the current mirror output to ensure proper operation (the compliance voltage) and ends at the upper voltage where the mirror goes out of specification.Ĭurrent mirror operation can be susceptible to changes in the biasing voltage, changes in temperature, as well as transistor matching, layout considerations, and other factors.Ī primary use of current mirrors is biasing amplifiers in integrated circuits.AC output resistance governs how much the output current varies with respect to the voltage changes.Output current magnitude for current sources.The primary current mirror specifications include: They can be implemented as an inverting current amplifier or as a current-controlled current source (CCCS). This FAQ reviews several common current mirror topologies and where they are used.Ĭurrent mirrors have high output impedance to help keep the output current constant regardless of loading and low input impedance to keep the input current constant irrespective of the drive. In some instances, the input current can be a varying signal, in which case the ‘copied’ current has the same variable characteristics. If the input current is constant, the ‘copied’ current is also constant, regardless of loading. A current mirror copies a current through one active device (such as a BJT or MOSFET) by controlling the current in another active device. ![]() Current mirrors are common circuits but are mostly invisible, except to designers of analog or mixed-signal integrated circuits who can use large numbers of current mirrors on a single chip.
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