Current Source is Modified into a Current Conveyor I. M. Filanousky, University of Alberta e circuit shown in Fig. l a is usually used as the current source for provid- ing a current, 21, that is independent on VBE(ON) voltage (see "Designer's Case- book' in Circuits & Devices, Vol. 9, No 2). Indeed, the current through the resistor, R, (Fig. la) is equal to: v -v, 21 =- R If the npn transistors 43 to Q6 are matched, the currents in the branches of the current mirror, Q3, Q4, should be equal, and the current in the load resistor, RL, will be equal to 21 as well. Instead, one may bias this circuit by two current sources (each of 21 value), as shown in Fig. lb. Then one can use the base of Q1 and the collectors of Q5 and Q6 as the circuit input and output ports. Also, the emitter of Q2 can be used as the circuit third-port. If the transistor's beta is high, then the small signals applied to these ports are described by the following relation- ships: i, =O U, =U, I, = I, . . The last equation is valid because the current mirror functions at ac signals as well. This system of equations can be rewrit- ten in the matrix form: which is frequently used in circuit theory. This matrix representation indicates that the circuit of Fig. l b is a second- generation current conveyor (CCII). This element is useful in synthesis of active fil- ters and high-speed amplifiers. Perhaps, the bipolar technology is not the best for realization of the obtained CCII (pnp transistors are usually slow in most bipolar technologies). However, now one can use this circuit as a proto- type for, say, CMOS technology and intro- duce other improvements (for example, using cascade current mirrors instead of simple ones). Substituting bias voltages by bias currents is used rather frequently in circuit design. But the small signal analysis of the obtained circuit, after the substitution, is usually forgotten. As one can see from this work, such analysis may be an interesting design trick that leads to unusual but useful circuits. CD

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Current Source is Modified into a Current Conveyor

I. M. Filanousky, University of Alberta

e circuit shown in Fig. l a is usually used as the current source for provid- ing a current, 21, that is independent

on VBE(ON) voltage (see "Designer's Case- book' in Circuits & Devices, Vol. 9, No 2). Indeed, the current through the resistor, R, (Fig. la) is equal to:

v -v, 21 =- R

If the npn transistors 43 to Q6 are matched, the currents in the branches of the current mirror, Q3, Q4, should be equal, and the current in the load resistor, RL, will be equal to 21 as well.

Instead, one may bias this circuit by two current sources (each of 21 value), as shown in Fig. lb. Then one can use the base of Q1 and the collectors of Q5 and Q6 as the circuit input and output ports. Also, the emitter of Q2 can be used as the circuit third-port.

If the transistor's beta is high, then the small signals applied to these ports are described by the following relation- ships:

i, = O

U , = U ,

I , = I , . .

The last equation is valid because the current mirror functions at ac signals as well.

This system of equations can be rewrit- ten in the matrix form:

which is frequently used in circuit theory. This matrix representation indicates that t h e circuit of Fig. l b is a second- generation current conveyor (CCII). This element is useful in synthesis of active fil- ters and high-speed amplifiers.

Perhaps, the bipolar technology is not the best for realization of the obtained CCII (pnp transistors are usually slow in most bipolar technologies). However, now one can use this circuit as a proto- type for, say, CMOS technology and intro- duce other improvements (for example, using cascade current mirrors instead of simple ones).

Substituting bias voltages by bias currents is used rather frequently in circuit design. But the small signal analysis of the obtained circuit, after the substitution, is usually forgotten. As one can see from this work, such analysis may be an interesting design trick that leads to unusual but useful circuits. CD