6.8 Transistor Switches with Complex Loads – Pulse and Digital Circuits

6.8 TRANSISTOR SWITCHES WITH COMPLEX LOADS

Transistor switch may not necessarily have a resistive load always. Depending on the requirement, the load can either be inductive or capacitive. A transistor switch with inductive and capacitive loads is studied in the sections below.

6.8.1 Switches with Inductive Loads

Consider a transistor switch, shown in Fig. 6.44 for which the load is an inductor, L in parallel with R. As the dc resistance offered by L is negligible, to ensure that the current in the transistor does not abruptly rise to a very large value when the transistor is driven into saturation, a resistance RC is added in series with the combination to limit the saturation current.

FIGURE 6.44 Switch with inductive load

The gating signal vi is a pulse train and the resultant output of the switch is shown in Fig. 6.45. When the gating signal is at V, Q is OFF. The voltage at the collector is VCC at t = 0. However, prior to this, the switch was ON and there was an inductor current IL. Suddenly, when the switch is driven into the OFF state, the inductor current does not become zero but flows through R and decays exponentially with a time constant L/R. Consequently, a spike of magnitude ILR appears across the inductor. Thus, momentarily there is a large reverse-bias voltage at the collector (= VCC + ILR) with respect to the base. It must be ensured that this voltage is not more than the breakdown potential of the collector diode. For this, R must be small. If R is small, the spike takes a longer time to decay. Alternately, if R is large, time constant becomes small, which allows the spike to decay faster. However, at the same time the magnitude of the spike = ILR becomes larger. Therefore, one has to strike a compromise in choosing R. The equivalent circuits that will enable us determine the output, when Q is OFF and when Q is ON are shown in Figs.6.46 and 6.47.

FIGURE 6.45 The input and output waveforms of a transistor switch with inductive load

FIGURE 6.46 The equivalent circuit when Q is OFF

FIGURE 6.47 Equivalent circuit when Q is ON

Again, at t = t1, Q goes ON and as the inductor will not allow any sudden change in the current, it behaves as an open circuit. If IO [= VCC/(RC + R)] is now the current through RC, a negative spike of IORC is developed and it decays exponentially with a time constant L/R where R is the parallel combination of R and RC. Now if a damper diode, D (in place of R) is connected across L, only positive spikes over and above VCC are available at the output, as shown in Fig. 6.48. If the polarity of the diode is reversed, only negative spikes are available at the output (see Fig. 6.49).

6.8.2 Switches with Capacitive Loads

A transistor switch with capacitive load is shown in Fig. 6.50. Once again the gating signal is a pulse train and the waveforms are shown in Fig. 6.51.

FIGURE 6.48 A switch with damping diode to derive positive spikes at the output

FIGURE 6.49 A switch with damping diode to derive negative spikes

FIGURE 6.50 A transistor switch with capacitive load

FIGURE 6.51 The waveforms of a transistor switch with capacitive load

The working of the circuit is explained as follows: Prior to t = 0. (i.e., at t = 0−), the input gating signal drives the transistor into saturation. As a result, the collector current is IC. The voltage at the output vo = VCE(sat).

However at t = 0+, the input drives the switch into the OFF state, the resultant equivalent circuit is as shown in Fig. 6.52.

When Q is turned OFF, the capacitor begins to charge to VCC. The voltage vo at t = 0+ was VCE(sat) and now as the capacitor charges, the output rises exponentially and ultimately reaches VCC. During this period IC = 0 as Q is OFF. At t = T1 the input forward -biases the emitter diode and Q is driven into active region resulting in a collector current IC = hFEIB = Io. Since Q is ON, the charge on CS discharges exponentially as shown in Fig. 6.53.

The moment the voltage at the collector forward-biases the collector diode, Q goes into saturation, Io falls to IC and vo returns to VCE(sat), as shown in Fig. 6.51. The output can be a near square wave by using collector catching diodes D1 and D2, as shown in Fig. 6.54.

The output voltage of the transistor switch with capacitive load (with and without collector catching diodes) is as shown in Fig. 6.55. Diode D1 conducts when the voltage at the collector is more than V1 by Vγ. As a result, the output is V1. Similarly, diode D2 conducts when the collector voltage is less than V2 by Vγ. As a result, the output is V2. The output is a near square wave.

FIGURE 6.52 The output circuit when Q is OFF

FIGURE 6.53 The equivalent circuit when Q is ON and is in active region

FIGURE 6.54 A transistor switch with capacitive load using collector catching diodes

FIGURE 6.55 The output of the switch with collector catching diodes

SOLVED PROBLEMS

Example 6.14: For the given common-emitter circuit shown in Fig. 6.56, calculate the minimum base current to achieve saturation for (i) RC = 33kΩ and (ii) 3.3 kΩ. Given VCE(sat) = 0.3 V and hFE = 60.

FIGURE 6.56 The given CE circuit

Solution:

(i) RC = 33 kΩ

Given hFE(min) = 60

(ii) RC = 3.3 kΩ

Example 6.15: For the CE circuit shown in Fig. 6.57 with VCC = 15 V and RC = 1.5 kΩ, VCE(sat) = 0.3 V. Calculate(a) the transistor power dissipation in cut-off and saturation; and (b)Power dissipation in the load.

FIGURE 6.57 The given CE circuit

Solution:

When the transistor is in cut-off, the power dissipation is zero. When the transistor is in saturation, apply KVL to the output circuit:

Power dissipation in the transistor = VCE(sat)IC = 0.3 × 9.8 × 10−3 = 2.94 mW.

Power dissipation in the load

SUMMARY
  • Semiconductors and Zener diodes, transistors and FETs can be used as switches.
  • A semiconductor diode switch is ON when forward-biased and OFF when reverse-biased.
  • Avalanche and Zener diodes are operated as switches when reverse-biased. As long as the reverse-bias voltage is less than VZ, the breakdown potential, the switch is OFF; and when the reverse-bias voltage is VZ, it is a closed switch.
  • Zener breakdown occurs due to the physical rupturing of covalent bonds on account of a strong electric field and avalanche breakdown is due to the avalanche multiplication of electron–hole pairs, aided by the external field.
  • The reverse recovery time of a semiconductor diode is the time taken for the reverse current to fall to 10 per cent of its forward current value when the device is suddenly switched from the ON state into the OFF state.
  • The breakdown voltages of avalanche and Zener diodes are temperature dependent. For a Zener diode, the breakdown voltage decreases with increase in temperature (negative temperature co-efficient) whereas for an avalanche diode, the breakdown voltage increases with temperature (positive temperature co-efficient).
  • For a current step of a small value, the diode is represented as a combination of resistance and capacitance.
  • For a current step of a larger magnitude, the diode is represented as a combination of resistance and inductance.
  • A transistor switch is said to be ON (saturation) when both the collector and emitter diodes are forward-biased.
  • A transistor switch is said to be OFF (cut-off) when both the collector and emitter diodes are reverse-biased.
  • Rise time is the time taken for the collector current to rise from 10 per cent of its final value to 90 per cent of its final value.
  • Fall time is the time taken for the collector current to fall from 90 per cent of its initial value to 10 per cent of its initial value.
  • The sum of delay time and rise time is called the turn-on time of the transistor.
  • The sum of storage time and fall-time is called the turn-off time of the transistor.
  • A commutating condenser can be used to reduce turn-on and turn-off times of a transistor.
  • hFE of a transistor is a function of temperature and collector current.
  • The transistor can be used as a latch.
  • To reduce the turn-off time of a transistor, it is preferable to reverse-bias the collector diode by a small voltage.
MULTIPLE CHOICE QUESTIONS
  1. A diode when forward-biased behaves as an:
    1. Open switch
    2. Closed switch
    3. Toggle switch
    4. None of the above
  2. A diode when reverse-biased behaves as an:
    1. Open switch
    2. Closed switch
    3. Toggle switch
    4. None of the above
  3. A transistor switch is said to be in the OFF state when the:
    1. Collector and emitter diodes are reverse-biased
    2. Collector diode is reverse-biased and the emitter diode is forward-biased
    3. Collector and emitter diodes are forward-biased
    4. Collector diode is forward-biased and the emitter diode is reverse-biased
  4. A transistor switch acts as a perfectly closed switch when operated in the:
    1. Cut-off region
    2. Saturation region
    3. Active region
    4. None of above
  5. The turn-on time of a transistor is given as:
    1. td + tr
    2. tr + tf
    3. ts + tf
    4. tr + ts
  6. The turn-off time of a transistor is given as:
    1. td + tr
    2. tr + tf
    3. ts + tf
    4. tr + ts
SHORT ANSWER QUESTIONS
  1. Briefly explain how a diode can be used as a switch.
  2. What is meant by the reverse recovery time of a diode?
  3. Distinguish between avalanche and Zener breakdowns.
  4. Define the switching times of a diode.
  5. A transistor is required to be operated in the active region. How do you bias the device?
  6. A transistor is required to be operated in the cut-off region. How do you bias the device?
  7. A transistor is required to be operated in the saturation region. How do you bias the device?
  8. Define the switching times of a transistor.
  9. A Zener diode has a breakdown potential VZ = 6.8 V at 25 °C. Calculate VZ at 75 °C if αZ = −0.1 per cent/°C.
  10. How does a commutating condenser improve the switching speed of a transistor switch?
  11. Briefly discuss the influence of breakdown voltages on the choice of supply voltage in a transistor switch.
  12. What do you understand by latching in a transistor switch?
  13. What is a non-saturating transistor switch?
LONG ANSWER QUESTIONS
  1. Explain with the help of suitable waveforms the switching times of a diode switch. Derive the expression for reverse recovery time.
  2. Explain with the help of neat circuit diagrams and waveforms, the method of improving the switching speed of a transistor switch by using a commutating condenser.
  3. Derive the expression for the rise time of a transistor switch.
  4. Derive the expression for the fall time of a transistor switch.
  5. Discuss the effect of temperature on the saturation parameters of the transistor.
  6. Explain what you understand by latching in a transistor switch.
  7. Write short notes on:
    1. A transistor switch with inductive load,
    2. A transistor switch with capacitive load, and
    3. A non-saturating transistor switch.
UNSOLVED PROBLEMS
  1. Calculate the output levels for the inputs 0 and −5 V to the circuit shown in Fig. 6p.1 and verify that the circuit is an inverter. Find the minimum value of hFE required. Neglect the junction saturation voltages. Assume ideal diode.
  2. For a common-emitter circuit VCC = 15 V, RC = 1.5kΩ and IB = 0.3 mA.
    1. Determine the value of hFE for saturation to occur.
    2. Will the transistor saturate if RC = 250 Ω?
  3. For a transistor, VCB = 50 V, Mn = 10, n = 8, hFE = 100. Calculate (a) VCBO(max) and (b) VCEO(max).

    FIGURE 6p.1 The given transistor switch

  4. Design the CE transistor switch shown in Fig. 6p.4, operating with VCC = 20 V and −VBB = −20 V. The transistor is expected to operate at IC(sat) = 5 mA, hFE = 25, VCE(sat) = 0, VBE(sat) = 0 and R2 = 4 R1. Determine the values of resistors RC, R1 and R2.

    FIGURE 6p.4 The given CE transistor switch

  5. Design a CE transistor switch shown in Fig. 6p.5, operating with VCC = 25 V and −VBB = −25 V. The transistor is expected to operate at IC(sat) = 4 mA, IB(sat) = 0.3 mA, hFE = 25, VCE(sat) = 0, VBE(sat) = 0 and R2 = 2R1. Determine the values of resistors RC, R1 and R2.vi varies from 0 to VCC.

    FIGURE 6p.5 The given CE transistor switch

  6. For the circuit in Fig. 6p.6, the input is a pulse of 16 V and duration T = 5 µs, fT is 10 MHz and fI = 1 MHz, αN0 = 0.99 and αNI = 0.5, CTC = 5 pF and hFE = 100, VBE(sat) = VCE(sat) = 0. Calculate (a) the turn-on time (b) turn-off time and (c) the time for which the switch is ON.

    FIGURE 6p.6 The given transistor switch

  7. For the circuit shown in Fig. 6p.7, the input is a pulse of 15 V and duration T = 1µs, fT is 5 MHz and fI = 1 MHz, αNO = 0.99 and αNI = 0.5, CTC = 5 pF and hFE = 100. Calculate (a) the turn-on time (b) turn-off time and (c) the time for which the switch is ON.

    FIGURE 6p.7 The given transistor switch

  8. For a transistor, VCB = 40 V, Mn = 5, n = 4, hFE = 100. Calculate (a) VCBO(max) and (b) VCEO(max).