**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 CircuitsĀ that has the base of transistor Q_{2} directly coupled to the collector of Q_{1}.

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 Q_{2}. This is done by estimating satisfactory levels of V_{E1} and V_{CE1}Ā for transistor Q_{1}. Then,

As discussed already, appropriate dc voltage levels for small-signal amplifiers are V_{CE}Ā = 3 V and V_{E}Ā = 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 (R_{5} in Fig. 12-20) should be much smaller than the external load resistor. Once R_{5} is selected, I_{C2} can be calculated using the voltage V_{R5} already determined. If I_{C2}Ā looks too small for satisfactory operation of the transistor, a suitable current level should be selected and a new value of R_{5} calculated. The collector current of Q_{1} is determined by making I_{C1},Ā very much greater than the base current for Q_{2}. This is done to ensure that I_{B2} has a negligible effect on the bias conditions of Q_{1}. Normally, just makingĀ I_{C1Ā }equal to I_{C2}Ā 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 hĀ-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 Q_{2} 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}Ā = 3 V, the base voltage for Q_{2} (measured from +V_{CC}) is,

The use of complementary transistors in Fig. 12-22 reduces the voltage drop across the emitter resistor of Q_{2}, compared to the situation in Fig. 12-20. This makes more voltage available for dropping across the Q_{2} 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.