Where I s1 is the reverse saturation current of this transistor Q1. That means here, voltage: V (BE (ref) )= V BE1= V BE We have seen, this collector current is the function of the voltage V BE. Because the base of both transistors are connected together and the emitter terminal of each transistor is grounded. So, here this voltage V BE(ref) and the voltage V BE 1 are the same. So, let’s call this second transistor as Q1. Now, let’s apply this voltage V BE to the base of the second transistor. So as you can see, here the voltage V BE is the function of the reference current. So this Is (ref) is the reverse saturation current of the reference transistor. So, in this case, we can say that this I ref or the collector current is equal to: Now, we already know the relationship between the collector current and the voltage V BE. That means here, we can assume that this collector current I C is approximately equal to I ref. Now for a moment let’s assume that this base current I B is negligible. This reference current is applied at the collector terminal. So, basically, it is the diode connected transistor. So as you can see, the collector and the base terminal of the transistor are connected together. So, this is the way, by which we can convert the reference current into the voltage. We need to apply this voltage between the base and the emitter of this transistor, so that, this collector current I C is proportional to the reference current. So, to generate this Current Mirror, somehow, we need to sense this reference current and we need to convert this current into the voltage. That means by controlling this voltage V BE we can control this collector current. Because the collector current I c is the function of the voltage V BE. Now, we know that the BJT can be used to generate the current source. So, now let’s see, how it can be designed using the BJT. I copy= I ref Or it could be equal to: I copy= N × I ref But in any case, it is as stable as the reference current source. That means this I copy can be equal to I ref. Now, here this I copy could be the same as the reference current or it could be either a multiple or the fraction of the reference current. So, this Current Mirror is the active circuit which senses the reference current and generates the copy or the number of copies of this reference current with the same characteristics. These current sources can be used to bias the amplifiers. That means this current source is as stable as the reference current source. So, this replicated current source has the same characteristic as the reference current source. So, instead of designing a stable current source for each amplifier, what is actually done is, one stable current source is fabricated in the integrated circuit itself and using the Current Mirror circuit, the same current source is replicated. So, for the amplifiers of the integrated circuits, we required such stable current sources. Well, fortunately, it is possible to design such current sources, but it involves a lot of circuitry. So, this current source should be able to maintain the constant current even if there is a change in the supply voltage. For example, if we take the case of the smartphone, then as the battery discharges then there is a reduction in the supply voltage. That means the value of this biasing current should not change with the temperature as well as the supply voltage. So, for that, this biasing current should be very stable. Moreover, by increasing the value of R b, we can increase the input impedance. Now, the advantage of biasing the amplifier with the current source is that here the emitter current is independent of the beta as well as R b. So, if we take any integrated circuit, then it consists of so many amplifiers and to bias these amplifiers, we required so many current sources. So, first of all, let’s understand what is Current Mirror and why it is used and for that, let me start with the current source. In this article we will learn about the Current Mirror circuits. Generation of the multiple copies of the Reference current:.
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