Test Transistors Fast
February 1969 Radio-Electronics

February 1969 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

By the time this "Test Transistors Fast" article appeared in a 1969 issue of Radio-Electronics magazine, the use of vacuum tubes in new designs had been relegated to the domain of hobby and specialized circuits. Very high power and frequency applications were still waiting for semiconductors capable of servicing such equipment, but the electronics industry, particularly the domestic consumer world, used transistors and what was available in the form of integrated circuits. While designers had already been using transistors for a decade or more, many of the aforementioned hobbyists and electronics repair servicemen were still getting accustomed to the newfangled technology and welcomed the publishing of these kinds of topics. In 1969, the only thing I knew about transistors was that they must have somehow been associated with the "transistor radio" I carried in my jacket pocket. It was too small to contain any of those glowing tubes we still had in our household TV set and tabletop radios (and in the dashboard of our family's 1960 Rambler auto for that matter), so surely there was a difference. If you are new to transistors because of just now getting into the field of electronics, then even though written half a century ago, the information is still relevant.

Test Transistors Fast - ABC's of Transistors

Learn how ohmmeters and transistor testers show solid-state faults. Study new age and horizontal circuits

By Sylvania Technical Staff

Ohmmeter tests will weed out definitely defective transistors. Since a transistor can be considered as two diodes connected back-to-back, the equivalent circuit (Fig. 1) is a guide to making simple resistance tests. For example, you should find a reasonably high front-to-back ratio between emitter and base, as well as between collector and base.

If you find a low resistance when the negative ohmmeter lead is connected to the emitter and the positive lead to the base, the transistor is an npn type. A high resistance here indicates a pnp type.

You may or may not measure a substantial front-to-back ratio from emitter to collector, even if the transistor is in good condition. A few transistors are symmetrical, meaning that the emitter and collector junctions have identical characteristics. For such devices the front-to-back ratio from emitter to collector is unity. On the other hand, most transistors are unsymmetrical, and you will measure a definite front-to-back ratio from emitter to collector although this ratio is usually lower than the other two ratios. Ohmmeter measurements on a "good" transistor selected at random are noted in Fig. 1.

Junction Resistances

Simple transistor testers are basically ohmmeters. A common follow-up test is to measure junction back resistances in terms of current flow. Nearly all testers provide an I_CBO test, which measures leakage current. It applies a moderate reverse voltage between collector and base with the emitter open, as in Fig. 2. A microammeter in the collector circuit indicates whether the back resistance of the collector junction is acceptably high. Manufacturers list I_CBO.

An I_CES test is made the same way, except that the emitter is shorted to the base. The I_CES current flow is normally somewhat greater than I_CBO current flow. Transistors are customarily rated for I_CES as well as I_CBO. Simple transistor testers seldom provide an I_CES test.

An I_ECS test is basically the same as an I_CES test, except that emitter and collector terminals are reversed. The reverse emitter-junction current is measured with the collector shorted to base. In a truly symmetrical transistor, I_ECS and I_CES readings are normally the same.

Many transistor testers do provide an I_CEO test. An I_CEO reading is normally greater than an I_CBO reading because the reverse current is increased by emitter-base junction action. This test, as well as the foregoing tests, are provided by instruments with maximum flexibility for de tests.

Alpha and Beta

A more critical test of a transistor is to measure its alpha or beta (see Fig. 3). Basically, this is a measurement of transfer resistance. As shown in Fig. 3-a, the emitter is forward-biased, and the collector is reverse-biased. A forward emitter-bias lowers the emitter junction resistance, which is transferred by lattice action as a lowering of the collector junction resistance. Alpha is defined as the ratio of collector-to-emitter current. Beta is defined as the ratio of collector-to-base current of the transistor.

The simple ratio of input to output currents gives the DC (static) alpha or beta of a transistor. This is a test that simulates large-signal operation of a transistor, where the input signal drives the transistor over a large excursion of its characteristics. On the other hand, transistors are commonly rated for small-signal operation, and published values of alpha and beta must be checked by a slightly modified procedure.

Small signal alpha and beta

To check small-signal alpha and beta, input current is set to an operating value specified by the transistor manufacturer. This establishes an operating point at a median in the characteristics, as might be used for class-A amplification. Then, input current is increased by a small amount. The corresponding change in collector current is noted. The small-signal alpha or beta is then given by the ratio of input-current change to output-current change. Alpha and beta are simply related to each other, as in Fig. 3.

The DC alpha or beta will not be the same as the small-signal alpha or beta because the transfer resistance of many transistors is nonlinear (Fig. 4). Note that a base-current change from 0 to 0.3 ma produces a much larger collector-current change than a base-current change from 0.9 to 1.2 ma. Hence, this transistor will measure a larger DC beta than its small-signal beta.

Transistor Circuit Applications

Automatic gain control: The age system develops a DC voltage in a tv receiver that has a direct relationship to signal strength. This voltage controls the gain of RF and video i.f. stages to limit picture variations due to changes in signal strength. For the most accurate age, the system is referenced to the portion of the signal which does not vary as a function of video information. These systems ere keyed or gated and referenced to the signal during horizontal sync pulse time, which represents 100% carrier power of the input signal. To reference at any other time could produce an agc voltage that would vary as the picture content varied; i.e., more age voltage on black scenes and less age voltage on white scenes, etc. Also the effects of noise on the age system should be minimized.

In most receivers, the chassis operates at maximum sensitivity when no signal is being received. When a signal is received, a voltage is developed that is proportional to the strength of the incoming signal. This agc voltage is used to control receiver gain. In a conventional tube-type tuner, a negative voltage controls tuner gain. Deviation occurs when gain reduction is applied to the video i.f. of solid-state sets. Here, gain reduction is handled by increasing the forward bias, which increases collector current in the transistor. Since a positive-going voltage is required to increase the npn transistor's forward bias, the negative age voltage is used to control an agc amplifier stage which. in turn, produces a positive voltage change at the base of the i.f. transistor controlled by agc. When an incoming signal becomes stronger, the negative age voltage increases and increases in the positive voltage on the base of the i.f. transistors (reducing i.f. gain).

The basic principle involved in using an increase in forward bias to reduce amplifier gain is shown in Fig. 5.

In some sets special circuits are used so that above a certain level, the i.f. and tuner receive different bias voltage ratios. The signal compared to bias for both i.f. and tuner is shown in Fig. 6.

Different voltages are used for the tuner and i.f. to keep the tuner in its best response range. The graph also shows noise and tweet extremes in tuner agc levels.

Signal strength is determined by "measuring" the horizontal sync pulse amplitude. This is the conventional means with keyed age systems. The average video level is not used since scene changes would appear as signal strength changes.

Composite video is taken from the collector of the first video amplifier. The signal at this point is positive sync. This signal is applied to the base of the age keyer. Since the age keyer is an npn transistor, the positive sync tends to make it conduct (see Fig. 7).

The keyer has the agc control in the emitter leg, which changes the gain of the keyer.

The agc keyer has no collector supply except for a positive pulse during the horizontal sync time. A diode connected in series with this pulse prevents the keyer from conducting except when the positive pulse is present. The agc coil also has a negative excursion which would forward bias the keyer's collector and destroy the transistor.

The positive keying pulse is supplied from a coil on the fly back transformer, the return leg of the agc amplifier. The amount of current drawn from the agc amplifier base circuit is proportional to the amount which the keyer is conducting which, in turn, depends on the sync pulse amplitude.

A capacitor in the base circuit of the agc amplifier charges to some value through R1. The higher the sync pulse amplitude, the greater the drain on the capacitor's charge. The less the charge on the capacitor, the less the agc amplifier conducts.

We then have a system which causes the agc amplifier to conduct more as the signal weakens and conduct less as the signal increases. Consequently, under maximum signal conditions the agc amplifier is driven toward cutoff and the collector voltage is at maximum. Maximum positive agc causes amplifier gain.

The collector voltage is fed directly to the tuner age line. Under maximum signal conditions, the i.f. age voltage is divided first by R2 and R3 and then by R4 and R5. Since conduction through R3 and R4 is light, the reverse voltage difference across the diode is large and it remains non-conducting.

As the signal weakens, the agc amplifier begins to conduct, which causes the reverse voltage differential on the diode to decrease. At some point, the diode conducts and places almost the same age voltage on the i.f. as the tuner.

Horizontal Deflection

The horizontal deflection requirements for a TV set call for a magnetic field capable of linearly deflecting the beam across the tube face and returning it to a starting point 15,750 times per second. An equivalent form of deflection circuit is in Fig. 8.

The transistor acts as a switch across the entire coil and power supply network. If the transistor is switched on, the yoke coil's current increases at about a linear rate. When a set time interval elapses the transistor is turned off. The inductive kick from the coil is impressed on the transistor diode and capacitor C. The coils and capacitor form a tank circuit and tend to oscillate or ring. The diode acts as a short or closed circuit on the opposite polarity of each "ring" cycle. By proper timing of transistor and diode switching, a sawtooth current passes through the coils. This current averages out to be near zero, provides circuit efficiency, and removes the need of beam decentering. Beam decentering means the need for its starting point to always be at one extreme side of the screen.

After the transistor is switched off and coil energy is transferred from the capacitor back to the coil, polarity reversal occurs and the damping diode conducts. The current is now flowing in the coils in the reverse direction and approaches zero. When zero is reached the transistor is again turned on, starting another cycle.

Horizontal Oscillator

The horizontal oscillator circuit in the Sylvania A02 chassis (Fig. 9) is an emitter coupled, two-stage multivibrator. Regenerative feedback voltage between stages is developed across the common emitter resistor.

When Q400 is at cut-off, collector voltage is about equal to battery voltage. This positive voltage is coupled to Q402 by C416, placing a forward positive voltage on the base of Q402. Q402 then goes to saturation and minimum collector voltage results. The positive voltage developed across R430 maintains a reverse bias on Q400, emitter to base. As the more negative base voltage of Q400 is neutralized (depending on the horizontal hold control setting) by the plus 12-volt supply voltage, the transistor goes back into conduction. Its collector voltage drops, applying a negative voltage to the base of Q402 and driving it to cut-off. This results in loss of emitter voltage on Q400, which is again biased off and the cycle repeats itself.

Horizontal output circuit

The horizontal oscillator input (in a Sylvania A02 chassis) to driver stage Q404 is approximately a square wave at about 3.8 volts peak-to-peak (see Fig. 10). It is amplified by the driver stage to a 25-volt peak-to-peak pulse. It is then applied to T400 to drive the final output Q406. This transistor normally conducts at saturation, and when the driver pulse is applied is cut-off almost instantaneously. This is a positive pulse from T400 and reverse biases the emitter base junction of Q406.

When Q406 stops conducting, the heavy B-plus current flowing through the transistor, through T402 winding (4 to 3 taps) and then through diode SC404, is cut-off. The resulting inductive "kick" by autotransformer action produces a 230-volt peak-to-peak spike on Q406's emitter and across the yoke coil network. The spike aids the driving pulse to Q406 from the driver stage through the low impedance path between emitter and base.

The SC409, C437 and R441 network across the yoke coils and C436 tends to clamp Q406's emitter to about a maximum of 240 volts peak-to-peak. This protects the transistor from breakdown due to voltages over this value. The clamping network is placed on the yoke only as a convenience.

The T402 secondary provides blanking, agc and high voltage spikes. SC411 rectifies the high-voltage spikes to provide 11kV to the picture tube.

Part of the autotransformer's primary is applied to diodes SC404 and SC406, which then act as voltage doublers to provide 120 volts. This voltage is used on the final video amplifier and G2 and G3 picture tube grids.

Damper diode SC407 is cut-off during positive excursions of the emitter spoke. When the negative excursions occur the circuit inductance tries to make the emitter voltage go negative. This is clamped to ground potential by the diode and will not go below 0 volts.

Capacitor C436, in series with the horizontal yoke, provides linearity correction for wide angle picture tube scanning. This capacitor causes de to pass through the flyback. A parabolic waveform is developed across the capacitor so that yoke voltage is less at the ends of the sweep than at the middle. This capacitor actually provides a series resonant current at about 5kHz to cause an S shaped current portion of a sine wave to flow. The S shaping capacitor is placed between the collector and emitter so yoke current does not have to flow through the power supply.

Operation of the horizontal output stage may be summarized as follows:

1. Output transistor has about 50% duty cycle.

2. Emitter voltage waveform is approximately the same as the base waveform.

3. C430 is a yoke retrace and boost capacitor.

4. The capacitor in series with the yoke is for linearity correction commonly referred to as "S" shaping. Peak-to-peak yoke current and fly back voltage is increased 10% by its use.

5. The flyback secondary uses third harmonic tuning. This provides a narrow secondary spike (10% of the cycle) and allows a lower peak-inverse voltage rating of the high voltage rectifier.

6. Third harmonic tuning is a function of leakage induction, winding spacing and distributed capacitance.

7. The average emitter DC voltage is 26 volts. This is actually a result of positive spikes 150 to 200 volts in amplitude. R-E

Posted March 17, 2023