BiCMOS Amplifiers

BiCMOS Amplifiers

• So far we have seen two basic amplifier technologies, the bipolar technology, which uses npn and pnp bipolar junction transistors, and the MOS technology, which uses NMOS and PMOS field effect transistors.

• An infinite input resistance.

• A large transconductance due to the bipolar transistor Q2.

• The bipolar junction transistors have a larger transconductance than MOS transistors biased at the same current level and they have higher switching speed. Due to large transconductance they provide larger voltage gains.

• On the other hand, MOS transistors have an essentially infinite input impedance at low frequencies and have very high packaging density. Due to almost infinite input impedance, MOS transistors have zero input bias current.

• The advantages of these two technologies can be exploited by combining bipolar and MOS transistors on the same substrate, i.e. in the same integrated circuit. Such technology is known as BiCMOS technology.

• We are familiar with the Darlington pair configuration of transistor. Fig. 8.2.1 (a) shows a modified Darlington pair configuration. It uses the bias current IBIAS/ or some equivalent element to control the quiescent current in transistor Qr The effective current gain of bipolar transistors is boosted in this circuit.

• Fig. 8.2.1 (b) shows a BiCMOS Darlington pair configuration. Here, transistor Qi is replaced with a MOSFET. This circuit has following advantages.

• As infinite input resistance

• A large transconductance due to the bipolar transistor Q₂.

Circuit Analysis

• Consider the small signal equivalent circuit shown in Fig. 8.2.2.

• Where g^c_m is the composite transconductance. The gm2 of the bipolar transistor is usually at least an order of magnitude greater than gm1 of the MOSFET. Therefore, the composite transconductance is approximately an order of magnitude greater  than that of the MOSFET alone. This circuit has following advantages.

• A large transconductance.

• An infinite input resistance.

• Fig. 8.2.3 (a) shows a bipolar cascade circuit, and Fig. 8.2.3 (b) shows a corresponding BiCMOS configuration.

Bipolar cascode configuration

• The output resistance of cascode circuit is very high.

• The input resistance looking into the emitter of Q2 is very low, thereby minimizing the Miller multiplication effect. Therefore, the cascode amplifier has a wider frequency bandwidth than the common-emitter circuit.

BiCMOS cascode configuration

• This circuit has the advantage of the infinite input resistance of M1.

•The frequency response of a BiCMOS cascode circuit is superior to that of an all MOSFET cascode circuit because the equivalent resistance looking into the emitter of a bipolar transistor is much less than the resistance looking into the source of a MOSFET.

2. Current Sources

• Cascade current sources increase the output resistance and the stability of the bias current. Fig. 8.2.4 shows a bipolar cascode constant current source.

• In this circuit, the output resistance is Ro =Pro4. Eventhough, the output voltage is varied, the bias current remains much more stable than the basic two-transistor current source.

• Fig. 8.2.5 shows a BiCMOS double cascode constant current source.

• The output resistance can be determined from the small signal equivalent circuit shown in Fig. 8.2.6.

• In this circuit,

• The gate voltage to M6 and the base voltage to Q2 and Q4 are constants. It is equivalent to signal ground.

• Vπ2 = 0, therefore gm6 Vπ2 = 0.

• The equivalent circuit can be rearranged as shown in Fig. 8.2.7.

• For the circuit shown in Fig. 8.2.7.

• The output resistance is extremely large.

• The output is given by,

R_o ≅ (g_m6r_o6) (β r_o4)

• In this circuit, the output resistance is increased by a factor (gmro6) compared to the bipolar cascode circuit in Fig. 8.2.4.

• If M₆ is to be replaced by a bipolar transistor, then a resistance rn 6 would be connected across the terminals indicated by Vgs6- This resistance would effectively eliminate the multiplying constant gm6ro6, and the output resistance would be essentially the same as that of the circuit shown in Fig. 8.2.4.

3. BiCMOS Differential Amplifier

• Fig. 8.2.8 shows a basic BiCMOS differential amplifier, with a constant-current source bias and a bipolar active load.

• The advantages of this circuit are,

• The infinite input resistance

• The zero input bias current.

• The disadvantage of this circuit is, since the MOSFET is used in the input stage, the offset voltage is relatively high compared to that of a bipolar input circuit. The offset voltages are generated because of the mismatching of differential pair input transistors.

4. BiCMOS Inverter

• The Fig. 8.2.9 shows the BiCMOS circuit. It consists of two bipolar transistors (T3 and T4), one nMOS and one pMOS transistor (both enhancement-type devices, OFF at V_in = 0 V).

• In this circuit the MOS switches perform the logic function and bipolar transistors drive output loads.

When V_in = 0 :

T₁ is OFF , therefore T3 is non-conducting.

T₂ is ON and supplies current to base of T₄

T₄ base voltage is set to V_dd.

• T₄ conducts and acts as current source to charge load CL towards V_dd. V_out rises to V_dd – V_be (of T₄) where Vbe (of T₄) is base emitter voltage of T₄. It is important to note that the pullup bipolar transistor T4 turns off as the output approaches 5V - V_be (ofT₄)

When V_in = V_dd

T₂ is OFF therefore T₄ is non-conducting.

T₁ is ON and supplies current to the base of T₃.

• T₃ conducts and acts as a current sink to discharge load CL towards 0 V.

V_out falls to 0 V + V_CE(sat) (of T₃). The _VCE(sat) (of T,) is saturation voltage from T3 collector to emitter.

• The T₃ and T₄ present low impedances when turned ON into saturation and load CL will be charged or discharged rapidly.

• The output logic levels will be good and will be close to rail voltages since VCE(sat) is quite small and V_BE ≈ 0.7 V. Therefore, inverter has high noise margins.

• Such inverter has high input impedance and low output impedance.

• The inverter also has high drive capability but occupies a relatively small area.

Review Questions

1. Describe the BiCMOS circuits of MOSFET.

2. Explain with neat diagram BiCMOS inverter.

3. Draw and explain the BiCMOS cascode amplifier.

4. Draw and explain the basic BiCMOS differential amplifier.