Current Mirror Article Index for
Current 		Website Links For
Current 		 

Information About

Current Mirror
  CATEGORIES ABOUT CURRENT MIRROR
   
analog circuits
   
APPAREL
BABY
BEAUTY
BOOKS
CAR TOYS
CELL PHONES
DVD'S
ELECTRONICS
GOURMET FOOD
GROCERIES
HEALTH & PERSONAL
HOME & GARDEN
JEWELRY
MUSIC
MUSIC INSTRUMENTS
OFFICE PRODUCTS
SOFTWARE
SPORTING GOODS
TOOLS & HARDWARE
TOYS
VIDEO GAMES
SHOPPING HOME
MORE SHOPPING...




BJT CURRENT MIRROR

s]]


Operation

Transistor Q1 is connected such that it behaves as a forward-biased Diode . The constant current through it (due to R1 and Vs) is determined mainly by the series resistance R1 as long as Vs is significantly larger than 0.7V, the typical forward VBE voltage for a silicon BJT . It is important to have Q1 in the circuit instead of a regular diode because, assuming the two transistors are closely matched, the base current for each transistor should be nearly identical since VBE for each transistor is identical. With nearly identical base currents, the matched transistors should then have nearly identical collector currents as long as VCE2 is not significantly larger than VBE. If VCE2 is much larger than VBE, the collector current in Q2 will be somewhat larger than for Q1 due to the Early Effect and further, Q2 may get substantially hotter that Q1 due to the associated higher power dissipation. When this occurs, the transistors will no longer be matched. To maintain matching, the temperature of the transistors must be nearly the same. In Integrated Circuit s and transistor arrays where both transistors are on the same die, this is easy to achieve. But if the two transistors are widely separated, the precision of the current mirror will not be stable.

Additional matched transistors can be connected to the same base and will supply the same collector current. In other words, the right half of the circuit can be duplicated several times with differing values of R2 on each. Note, however, that each additional right-half transistor "steals" a bit of collector current from Q1 due to the non-zero base currents of the right-half transistors. This will result in a small reduction in the programmed current.)


Circuit analysis


The current through R1 is given by:

I_{R1} = I_{C1} + I_{B1} + I_{B2}

Where I_{C1} is the collector current of Q1, I_{B1} is the base current of Q1, I_{B2} is the base current of Q2.

The collector current of Q1 is given by:

I_{C1} = \beta_0 I_{B1}

Where \beta_0 is the DC current gain of Q1. If Q1 and Q2 are perfectly matched, \beta of Q2 will be:

:\beta_2 = \beta_0\ (1 + rac{V_{CB2}}{V_A})
:where VA is the Early Voltage .

Because VBE1 = VBE2 and Q1 and Q2 are matched, IB1 = IB2.

After substituting and rewriting, the collector current in Q2 is given by:

I_{C2} = rac{I_{R1}}{1 + rac{2}{\beta_0}}\ (1 + rac{V_{CB2}}{V_A})

If \beta_0 >> 1, then

I_{C2} \approx I_{R1}\ (1 + rac{V_{CB2}}{V_A})

Typical values of \beta will yield a current match of 1% or better. Even better accuracy and precision can be achieved with more sophisticated current mirrors, such as the Widlar Current Source , Cascoded Current Sources and Wilson Current Source .


MOSFET CURRENT MIRROR

s]]
>


Operation

Transistor ''T''1 is operating in the Saturation region, and so is ''T''2. In this setup, the output current ''I''out will be directly related to ''I''ref. ''I''d is a function of the gate source voltage of a transistor given by ''I''d = ''f''(''V''gs). This is a relationship derived from the functionality of MOSFET technology. In the case of a current mirror, ''I''d = ''I''ref. Thus ''I''ref is a function of ''V''gs. ''I''ref is known and normally provided by a Band-gap Reference Circuit . By this same relationship we find ''I''out. ''I''out = ''I''d is also a function of ''V''gs. As we are able to derive ''V''gs from ''I''ref based on properties of the transistor, this same ''V''gs applies to transistor ''T''2 in the diagram. This principle works because both transistors ''T''1 & ''T''2 have good matching of their properties such as channel length and doping concentrations.
The source terminals of both transistors are also biased to the same voltage so that the ''V''gs property can be applied. At this point the relationship of ''f''(''V''gs) = ''I''out is applied thus finding that ''I''out = ''I''ref. ''I''s can also be shown that ''V''ds (drain-source voltage) of each transistor is the same. The ''I''d equation that describes this principle is –

I_{d} = rac{1}{2}K_{p}\left( rac{W}{L} ight)(V_{gs} - V_{t})^2 (1 + \lambda.V_{ds})

where,

K_{p} = \mu C

''µ'' and ''C'' are constants associated with the transitor, W/L is the width to length ratio of the transistor, ''V''gs is the gate-source voltage, ''V''t is the threshold voltage, λ is the channel length modulation constant, and ''V''ds is the drain source voltage.


SEE ALSO




REFERENCES