Common Source Power Amplifier Using an Op Amp Driver Stage:

Basic Circuit Operation – The Class-AB MOSFET Power Amplifier with OP Amp Driver Stage circuit in Fig. 18-45(a) consists of an operational amplifier (A₁), two MOSFETs (Q₃ and Q₄), and several resistors. The op-amp together with resistors R₄, R₅, and R₆ and capacitor C₂ constitutes a non-inverting amplifier. The two MOSFETs are a complementary Common Source Amplifier output stage.

The gate-source bias voltages for Q₃ and Q₄ are provided by the voltage drops across resistors R₇ and R₈, which are in series with the op-amp supply terminals. So, the op-amp supply currents (I_S(+) and I_S(-)) determine the levels of V_GS3  and V_GS4.

Suitable gate-source threshold voltages (V_GS(th)) to bias the output transistors for Class AB operation, and typical op-amp supply currents (I_S) can be determined from the device data sheets.

Figure 18-45(b) shows that the op-amp supply currents are largely the collector currents in the op-amp output stage BJTs (Q₅ and Q₆). Thus, R₇ and R₈ are collector resistors for Q₅ and Q₆. When the base voltage for Q₅ and Q₆ is increased in a positive direction, Q₅ collector current (I_S(+)) increases, causing V_GS3 to increase, and thus increasing the MOSFET drain current I_D3. At the same time, the Q₆ collector current (I_S(-)) decreases, reducing V_GS4 and drain current I_D4. The result I_D3 flows through R_L producing a positive output voltage swing (+V_o). Note that the op-amp output voltage is also positive at this time.

When the base voltage at Q₅ and Q₆ is negative-going, I_S(-) increases, causing an increase in V_GS4 and an increased level of I_D4. During this time, I_S(+) decreases, reducing V_GS3 and I_D3. Thus, I_D4 flows through R_L producing a negative output voltage swing (-V_o). The op-amp output voltage is also going negative during the time that Q₄ is creating the negative output voltage across R_L.

It is seen that BJTs Q₅ and Q₆ operate as common-emitter amplifier stages and that MOSFETs Q₃ and Q₄ function as Common Source Amplifier circuits. Both stages produce voltage gain which should be multiplied with the op-amp open-loop gain to determine the total open-loop gain for the circuit. Using typical quantities, the overall open-loop gain can be shown to be around 4 x 10⁶.

Returning to Fig. 18-45(a), the complete (basic) circuit operates as a non-inverting amplifier with a closed-loop voltage gain;

As explained, the power MOSFETs produce the high output current required by the amplifier load. An op-amp operating alone could not supply the load current.

Bias Control:

As previously discussed, the bias current in the output transistors of a MOSFET Power Amplifier with OP Amp Driver Stage should be adjustable. Control over the bias current flowing in transistor Q₃ in Fig. 18-47(a) might be achieved by using a variable resistor for R₇, as illustrated. This allows I_D3 to be adjusted by varying V_GS3. When I_D3 is increased, and the additional drain current flows through R_L, the dc feedback keeps the V_o equal to zero. The feedback produces the necessary change in V_GS4 to make I_D4 closely follow I_D3. Thus, adjustment of resistor R₇ controls the level of I_D4 as well as I_D3.

A disadvantage of using R₇ to adjust the bias current is that the voltage gains produced by Q₅ and Q₆ become unequal when R₇ and R₆ have different resistance values. This can be overcome by the negative feedback, however, the bias resistors can be kept equal by using the variable current source shown in Fig. 18-47(b).

In the variable current source, transistor Q₁ is biased from the emitter of Q₂, and the Q₂ base is connected to the collector of Q₁. The collector-emitter voltage of Q₁ is,

Assuming that I_B2 ≪ I_C1, the Q₁ collector current is,

With I_B1 ≪ I_E2, the Q₂ collector current is approximately,

Variable resistor R₃ controls the level of I_C2. Also.

As explained, the dc negative feedback causes V_GS4 to always be equal to V_GS3, So, R₃ controls the MOSFET gate-source voltages, and thus controls the quiescent drain current. The ac load offered to the collector of Q₅ [see Fig. 18-45(b)] is R₇, regardless of the Q₂ current. Consequently, the Q₅ and Q₆ stage gains are equal.

Output Voltage Swing:

One problem with the basic circuit in Fig. 18-45 is that the output voltage swing is limited by the voltage levels at the op-amp supply terminals. As illustrated in Fig. 18-49(a), the positive supply voltage to the op-amp is (V_CC – V_R7), and the negative supply voltage is -(V_EE – V_R8). Also, recall that V_R7 must be increased by ΔV_GS3 in order to drive the output in a positive direction, and V_RB must be increased by ΔV_GS4 to drive V_o negative.

In Eq. 18-29, I_p is the peak output voltage delivered to R_L and g_fs is the MOSFET forward transconductance. Thus, the minimum supply voltage levels at the op-amp terminals are,

The op-amp peak output voltage is normally limited to approximately 1 V below the voltages at the supply terminals, (although, as previously noted, rail-to-rail op-amp are available). Consequently, the output voltage swing from the circuit is likely to be less than ±(V_CC – 4 V), [see fig. 18-49(a)]. For greatest efficiency, it is necessary to drive the output as closely as possible to ±V_CC. This can be achieved by the circuit modification shown in Fig. 18-49(b).

Resistors R₉ and R₁₀ in Fig. 18-49(b) divide V_o, so that the op-amp output can be substantially lower than the peak output voltage developed across R_L. This means that the voltage levels at the op-amp supply terminals no longer limits the amplifier output voltage swing. Now, the largest peak output voltage that can be achieved is limited only by the supply voltage and R_D(on).

R₉ and R₁₀ also provide negative feedback that controls the gain of the stage made up of the op-amp output BJTs and the common source Amplifier. This is illustrated in Fig. 18-50. A further function of R₉ and R₁₀ is that they can be selected to limit the op-amp output current in the event of R_L becoming short-circuited.

Class-AB Common Source Power Amplifier:

The complete circuit of the Common Source Power Amplifier with OP Amp Driver Stage is shown in Fig. 18-52. Note the inclusion of resistors R₁₁ and R₁₂, and capacitors C₃ and C₄. Resistors R₁₁ and R₁₂ are typically 100 Ω. They have no effect on the circuit dc conditions, but they help to reduce the possibility of oscillations in the output stage. The additional stage of voltage gain constituted by the MOSFETs and the op-amp (common-emitter) output transistors increases the possibility of circuit instability. Capacitor C₃ helps to ensure frequency stability by acting with resistor R₉ to introduce a phase lead in the output stage feedback loop. The phase lead cancels some of the phase lag in the overall circuit.

The additional stage of amplification extends the high cut-off frequency of the amplifier above the cut-off frequency of the op-amp operating alone. If the op-amp (full power) upper cutoff frequency for an overall voltage gain of 20 (or 26 dB) is 200 kHz, and the additional stage has a gain of 2 (or 6 dB), the circuit cutoff frequency is 400 kHz. For audio applications, it is normal to include capacitor C₄ (see Fig. 18-52) which is usually selected to set the amplifier upper cutoff frequency around 50 kHz, or lower.