Constant current source in integrated operational amplifier

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Constant current source in integrated operational amplifier

1. Basic mirror current source 　　　　 　　 　　　　　　　　　　　　　　　　　　　　　　　 　　The circuit is shown in Figure 6.1. The parameters of T 1 and T 2 are exactly the same, that is, β 1 =β 2 , I CEO1 =I CEO2 . From the circuit, we can see that V BE1 =V BE2 , I E1 =I E2 , I C1 =I C2 　　 　　When β>>2,　　　 　　 　　I R =I REF is called the reference current. It can be seen from the above formula that when R is determined, I R is determined, and I C2 is also determined accordingly. We regard I C2 as the mirror image of I R , so Figure 6.1 is called a mirror image constant current source . Improved circuit 1: 　　　　　　 　　Figure 6.2 is a basic mirror current source with a buffer stage. It is an improvement on the shortcomings of the basic mirror current source. The difference between the two is that transistor T3 is added . Its purpose is to reduce the shunting effect of I B on IR of transistors T1 and T2 , improve the mirror accuracy, and reduce the impact of insufficient β value. 　　　　　　　　　　　　　　　　　　　　　　　　　 　　 Improved circuit 2: 　　　　　　 　　Figure 6.3 is a mirror current source with an emitter resistor, where Re1 = Re2 . The inputs of the two tubes are still symmetrical, so 　　　　　　　　　　　　　　　　　　　　　　　　 　　 If Re1 of this circuit is not equal to Re2 , then IC2 is proportional to the ratio of (Re1 , Re2 ) , therefore, this current source is also called a proportional current source. 2. Micro current source 　　　 　　　 　　　　　　　　　　　　　　　　　　　　　　　　 The circuit is shown in Figure 6.4. When IR is constant, I C2 can be determined as: 　　 It can be seen that I 0 can be controlled by using the base-emitter voltage difference V BE of the two tubes . Since the value of V BE is small, a small working current can be obtained by using a small resistance Re2 - a micro current source. Example: The circuit is shown in Figure 6.5. 　　　 Given that the parameters of BJT are the same, find the relationship between each current source and the reference current. 　　　　　　　　　　　　　　　　　　　　　　　　　　 　　 3. Main Application of Current Source - Active Load 　　As mentioned above, increasing R c can improve the voltage gain of the common-emitter amplifier circuit. However, R c cannot be too large, because the cost of manufacturing large resistors in the integrated process is too high, and, under the condition of constant power supply voltage, the larger R c is, the smaller the output amplitude is. So, can we find a component to replace R c , whose dynamic resistance is large, so that the voltage gain is increased, but the static resistance is small. So as not to reduce the output amplitude? Naturally, we can consider the transistor constant current source. Since the current source has the characteristics of small DC resistance and large AC resistance, it is widely used as a load in analog integrated circuits - active load, as shown in Figure 6.6.