Direct Coupled Circuits | Circuit Design

Direct Coupled Circuits:

For economy, the number of components used in any circuit should be kept to a minimum. The use of direct coupling between stages is one way of eliminating components. Figure 12-20 shows a Direct Coupled Circuits that has the base of transistor Q₂ directly coupled to the collector of Q₁.

Comparing this to the capacitor-coupled circuit in Fig. 12-15, it is seen that the two bias resistors for Stage 2 and the interstage coupling capacitor have been eliminated. This is a saving of only three components, however, the total savings can be considerable when many similar circuits are to be manufactured.

Circuit Design:

The first step in the design of the circuit in Fig. 12-20 involves determining a suitable level of base bias voltage for Q₂. This is done by estimating satisfactory levels of V_E1 and V_CE1 for transistor Q₁. Then,

As discussed already, appropriate dc voltage levels for small-signal amplifiers are V_CE = 3 V and V_E = 5 V, except in the case of a very low supply voltage.

As for all amplifier circuits, the resistor at the collector of the output transistor (R₅ in Fig. 12-20) should be much smaller than the external load resistor. Once R₅ is selected, I_C2 can be calculated using the voltage V_R5 already determined. If I_C2 looks too small for satisfactory operation of the transistor, a suitable current level should be selected and a new value of R₅ calculated. The collector current of Q₁ is determined by making I_C1, very much greater than the base current for Q₂. This is done to ensure that I_B2 has a negligible effect on the bias conditions of Q₁. Normally, just making I_C1equal to I_C2 is the simplest way to achieve the desired effect.

Circuit Analysis:

Analysis procedure for a two-stage Direct Coupled Circuits is similar to that for analysis of a two-stage capacitor-coupled circuit. The h-parameter equivalent for the circuit of Fig. 12-20 is exactly like Fig. 12-19, except that the bias resistors for the second stage are, omitted. The voltage gain and impedance equations are as determined for the capacitor-coupled circuit. Because the component numbers differ slightly for the two circuits, care must be taken in substituting components into the equations. For example, the voltage gain of the second stage of the circuit in Fig. 12-20 is,

Use of Complementary Transistors:

The Direct Coupled Circuits, two-stage circuit illustrated in Fig. 12-22 is similar to that in Fig. 12-20, except that transistor Q₂ is a pnp device. The transistors are selected to have similar characteristics and parameters although one is npn and the other is pnp. This means that they are complementary transistors.

Suppose the circuit in Fig. 12-22 is to be designed to use a 14 V supply, as in Example 12-8. Allowing V_E1 = 5 V and V_CE1 = 3 V, the base voltage for Q₂ (measured from +V_CC) is,

The use of complementary transistors in Fig. 12-22 reduces the voltage drop across the emitter resistor of Q₂, compared to the situation in Fig. 12-20. This makes more voltage available for dropping across the Q₂ collector resistor.

The design procedure for the circuit using complementary transistors is very similar to the procedure for designing the non-complementary circuit in Fig. 12-20.