2N4264

3

Motorola Small–Signal Transistors, FETs and Diodes Device Data

CURRENT GAIN CHARACTERISTICS

Figure 2. Minimum Current Gain

IC, COLLECTOR CURRENT (mA)

20

30

50

70

100

2.0

3.0

10

50

70

10

100

1.0

200

30

20

5.0

7.0

2N4264

VCE = 1 V

TJ = 125°C

25

°C

–15

°C

–55

°C

When a transistor is held in a conductive state by a base current, IB,

a charge, QS, is developed or “stored” in the transistor. QS may be

written: QS = Q1 + QV + QX.

Q1 is the charge required to develop the required collector current.

This charge is primarily a function of alpha cutoff frequency. QV is the

charge required to charge the collector–base feedback capacity. QX is

excess charge resulting from overdrive, i.e., operation in saturation.

The charge required to turn a transistor “on” to the edge of saturation

is the sum of Q1 and QV which is defined as the active region charge,

QA. QA = IB1tr when the transistor is driven by a constant current step

(IB1) and IB1 < <

IC

hFE

.

If IB were suddenly removed, the transistor would continue to

conduct until QS is removed from the active regions through an

external path or through internal recombination. Since the internal

recombination time is long compared to the ultimate capability of a

transistor, a charge, QT, of opposite polarity, equal in magnitude, can

be stored on an external capacitor, C, to neutralize the internal charge

and considerably reduce the turn–off time of the transistor. Figure 3

shows the test circuit and Figure 4 the turn–off waveform. Given QT

from Figure 13, the external C for worst–case turn–off in any circuit is:

C = QT/∆V, where ∆V is defined in Figure 3.

NOTE 1

Figure 3. QT Test Circuit

Figure 4. Turn–Off Waveform

+10 V

t1

∆V

<1 ns

3 V

270

Ω

8 pF

9.2 k

Ω

CS < 4 pF

C

COPT

TIME

C < COPT

C = 0

PULSE WIDTH (t1) = 5 µs DUTY CYCLE = 2%

0