Tics of Transistors the Emitter Is Not Quite As Posi- Tive, and the Collector Is Far More %

ana aescrmea how to troubleshoot I I I I transistorized circuits. Incorporated into the text was a description of a simple in-circuit transistor checker that utilized the X-Y display capabilities of an oscilloscope. la. NPN silicon transistor typical bias lb. PNP germanium transistor typical We have received many requests for conditions bias conditions copies of this article and especially Figure 1. Transistor bias examples information about the transistor showing the “on” condition checker. So, by popular demand, here is a repeat of “How to Troubleshoot In a PNP transistor, the base must saturated germanium transistor Transistorized Circuits Faster,” by be more negative than the emitter in may have as low as 0.05 volts be- George Stanley, based mostly on his order for current to flow. tween its emitter and collector, book, Transistor Basics: A Short So “on” bias voltages for a transistor while a saturated silicon transistor Course. Copyright @ 1967, 1975 by can be summed up by referring to may have 0.5 volts or less between Hayden Book Company, Inc. All Figure 1 and the following two rules: these leads. rights reserved. This material is - For a PNP, the base is negative, printed with the permission of 0 the emitter is not quite as nega- Hayden Book Co., Inc., Rochelle n, tive, and the collector is far more Park, N.J. 72 negative. 0 -76 0 Fundamental Characteris- - For an NPN, the base is positive, tics of Transistors the emitter is not quite as positive, and the collector is far more % Before describing specific trouble- positive. 0 0 shooting tips, let’s take a moment There is a distinct difference be- and review several important tran- tween a transistor being turned “on” Figure 2. Transistor bias example sistor characteristics. and being “saturated.” When a tran- showing the “saturated” condition 1 Conventional PNP or NPN transis- sistor is saturated, it’s generally tors are basically ccofl”devices and thought of as being almost a short, must be biased “on” to their operat- that is, the IR drop across the emit- ing point. This is done by forward ter and collector resistors equals the biasing the base-emitter diode to supply voltage as shown in Figure 2. make the transistor conduct. Refer Naturally this means that there is to Figure 1 for examples of forward practically no voltage drop between bias on NPN/PNP silicon and ger- the collector and emitter of the tran- nsistors. sistor. In this condition, both the base-emitter and base-collector di- Figure 1, an NPN tran- odes are forward biased (where in have its base more posi- the “on” condition only the base- le emitter in order for emitter is forward biased - the ow. base-collector is reverse biased). A WWIW&WMRGHIYWOM “Saturated” or “off” are the usual remove the forward bias as shown in amplifier stage, excessive leakage conditions found in digital circuits. Figure 3. The collector voltage current can cause clipping distortion In ac circuits where transistors are should then rise to the approximate because of the shift in the quiescent used as amplifiers instead of level of the supply voltage. (Any dif- operating point. switches, the amount the transistor ference is caused by ICO,the is turned on depends upon current collector-to-base leakage current.) Tip #3 gain (beta) of the transistor, the re- The higher the collector voltage In an amplifier with clipping dis- sistors in series with the collector rises, the lower ICO,and the better tortion, try cooling each transis- the transistor. and emitter, and the supply voltage. tor with spray coolant. Quite likely you will find that when the Basic Troubleshooting Tips I 1 f IOV 1 leaky transistor is cooled the clipping distortion disappears. In troubleshooting transistor cir- Conversely, heating a leaky tran- cuits, the most important area to sistor will make the problem examine is the base-emitter junction much worse by greatly increas- as this is the control point of the ing the ICOleakage. transistor. Basic Circuit Analysis If the base-emitter junction is forward biased, the transistor would An interesting problem is illustrated normally be “on.” Figure 3. Amplifier with forward bias removed in Figure 4. In this circuit, both transistors are of the NPN type. Note If the base-emitter junction has zero I I If the collector voltage doesn’t rise as that Q2 has 0.8 V reverse bias on its bias or reverse bias, it should be emitter-base junction, but the 2.0 turned off. If it is not off under these expected, we’ve identified a bad transistor. This technique is per- volts on the emitter means that there conditions, it is either shorted or ,- is 2 mA of emitter current. Now, leaky. fectly safe in AC coupled circuits. ”p However, in some DC coupled cir- since the emitter-base junction is not cuits, we could cause damage if shorted, this 2 mA of current also Tip #I base-emitter shorts are applied flows through the 8K resistor in the Measure the base-emitter volt- around high power levels (e.g., such collector of Q2. Therefore, the age. From this decide how the as the output stage of a power collector voltage, Vcc, is: transistor should be behaving. amplifier). 18V - (8K) x (2 mA) = 2V Then look at the collector voltage Thus, it would appear that Q2 has a and see if the transistor is behav- Now, back to ICO,the collector-to- short between collector and emitter. ing as it should be. base leakage current mentioned previously. As we implied, if the For example, if the base-emitter transistor was perfect it would have voltage is 0.6 volts forward biased no ICOleakage current. Look at Fig- and the collector voltage is the same ure la again. Note the collector volt- as the supply voltage, something is age is more positive than the base wrong. Probably the collector-base voltage. In this “on” condition the junction is open. base-collector diode junction is reverse biased. This reverse biased Expanding on the above idea leads diode should be off, but because we to our second troubleshooting tip. have never been able to make a perfect diode, there is a very small cur- Fig. 4. Direct coupled two-stage rent leaking across it. This leakage example circuit Tip #2 current flows through the collector- ~~ ~ Modify the control signals pre- base junction and part of it goes Another interesting problem in sent and see if the circuit re- through the base-emitter (control troubleshooting illustrated in sponds accordingly. point) junction. Figure 5. Although the emitter ‘ 3 current of Q1 is 1 mA, the collector For example, if the transistor is for- Since leakage current is extremely current is only 0.52 mA (i.e., ward biased as shown in Figure 1, temperature sensitive, we can use 5.2V+10K). Stage Q2 shows 5 mA see if it is behaving as an amplifier. this to our advantage in trou- flowing in both the emitter and Short the emitter to the base to bleshooting. For example, in an collector circuits, so Q2 is either WWW.HPARCHIVE.COM associated resistance and capaci- I I I I 0 Figure 6 shows a simplified schema- tance. The loop is caused by the tic of the transistor checker and the capacitance (probably a coupling Fig. 5. Capacitive coupled two-stage ideal voltage vs. current waveforms capacitor), and the fact that the example circuit you can expect to see. waveform is not a perfect "right" angle is because of the associated shorted or saturated. The one voltage resistance (probably bias or load that would answer this question is resistors). not given; i.e., the voltage on the base of Q2. If everything were working correctly, this voltage would be approximately 1.5V. t H"rll (15V) x (10K) VB = 90K + 10K Figure 6. Transistor checker and ideal waveforms VB = 1.5v Since the transistor checker puts out What appears to have happened is Figure 8. Ideal waveforms for a good a sine wave that has alternatively that C3 is shorted. This would ex- diode positive and negative half cycles, we' plain why there is only 0.52 mA flow- would expect a perfect diode to being through resistor R4. The other have as shown in Figure 7. 0.48 mA is flowing through C3 and resistor R6. If C3 were shorted, it NOTE: All references to a diode also would also explain the voltages on imply the base-emitter or base- Q2. The 5.2 V on the base produces collector diode junctions of a 5.0 volts on the emitter, which, in transistor. turn, causes the 5 mA of d-c current to flow and Q2 to saturate. In actual practice, the waveforms shown in Figure 7 are all possible If capacitor C3 were replaced, the because the test leads are not base voltage of Q2 would be 1.5 V dc, and the voltage on the emitter would be about 1.3 V dc. This, in turn, Figure 9. Typical in-circuit waveform for a good transistor would cause about 1.3 mA of dc to flow. The resultant collector voltage would be 12.4 V dc. The schematic of the tester shows a switch that shorts out a 5.6K resis- In-CircuitTransistor Tester tor. This switch is primarily for current limiting so you don't damage Even though all the above tips are sensitive transistors. You can also good ones, there is a transistor tester use it for in-circuit vs. out-of-circuit that will speed up troubleshooting testing.

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