Why is the current mirrors output unequal? - Electrical Engineering Stack Exchange Why is the current mirrors output unequal? - Electrical Engineering Stack Exchange

Pmos current mirror simulation dating, simulation shows the problem of current mirror mismatching?

Usually, two identical converters are used but the characteristic of the first one is reversed by applying a negative feedback.

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The simplest bipolar current mirror shown in Figure 1 implements this idea. Practical approximations[ edit ] For small-signal analysis the current mirror can be approximated by its equivalent Norton impedance.

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To eliminate these undesired disturbances, a current mirror is composed of two cascaded current-to-voltage and voltage-to-current converters placed at the same conditions and having reverse characteristics.

Thus a current mirror consists of two cascaded equal converters the first - reversed and the second - direct.

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It consists of two cascaded transistor stages acting accordingly as a reversed and direct voltage-to-current converters. Mirror characteristics[ edit ] There are three main specifications that characterize a current mirror.

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If Q1 and Q2 are matched, that is, have substantially the same device properties, and if the mirror output voltage is chosen so the collector-base voltage of Q2 is also zero, then the VBE-value set by Q1 results in an emitter current in the matched Q2 that is the same as the emitter current in Q1[ citation needed ].

Consequently, the voltage drop across Q1 is VBE, that is, this voltage is set by the diode law and Q1 is said to be diode connected. See also Ebers-Moll model.

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The current delivered by the mirror for arbitrary collector-base reverse bias, VCB, of the output transistor is given by: The first is the transfer ratio in the case of a current amplifier or the output current magnitude in the case of a constant current source CCS.

The emitter of transistor Q1 is connected to ground.

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By applying a negative feedback simply joining the base and collector the transistor can be "reversed" and it will begin acting as the opposite logarithmic current-to-voltage converter; now it will adjust the "output" base-emitter voltage so as to pass the applied "input" collector current.

However, an ideal current source is unrealistic in several respects: The second is its AC output resistance, which determines how much the output current varies with the voltage applied to the mirror. Its collector-base voltage is zero as shown.

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This minimum voltage is dictated by the need to keep the output transistor of the mirror in active mode. It is not obligatory for them to be linear; the only requirement is their characteristics to be mirrorlike for example, in the BJT current mirror below, they are logarithmic and exponential.

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There are also a number of secondary performance issues with mirrors, for example, temperature stability. The third specification is the minimum voltage drop across the output part of the mirror necessary to make it work properly.

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In large-signal hand analysis, a current mirror is usually and simply approximated by an ideal current source. The range of voltages where the mirror works is called the compliance range and the voltage marking the boundary between good and bad behavior is called the compliance voltage.