Transistors

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Transistors are three termainal devices where one terminal controls the power through the other two. They come in 2 different famlies Bipolar ( a little depressed, but only some of the time ) and Field Effect ( usually metal oxide semiconductors MOS). We have seperate sections for each.

Contents

Bipolar Transistors

TWO TYPES:

Bipolar transistors are current controlled devices. The current flowing in/out of base controls the current flowing in/out of the Collector. There are two basic types of Bipolar transistors: NPN and PNP.

In bipolars, the collector current (Ic) is controlled by the base current (Ib). Ic equals the Ib times a variable called "Hfe" (often listed in transistor datasheets). In other words, Ic = Ib * Hfe. Hfe can range from 30 (for big bipolars) to several hundred.

TERMINALS: Collector, Base, Emitter


NPN Operation: To turn on and get collector current (Ic) flowing, a small base current must be squirted INTO the base. Do this by raising base voltage through a base resistor. Current flows into collector and out of emitter. If this current is too small (or zero), the base is starved and collector current shuts off. When you squirt enough current into the base, the base-emitter acts like a diode. The way you know you are squirting enough current into the base is by checking the voltage between base and emitter. When transistor is on full, the base will be several hundred millivolts above the emitter voltage (< 500 mV if barely on, usually 600-900 mV when full on).

PNP Operation: To turn on and get Ic flowing, a small base current must be pulled OUT OF the base. Do this by pulling base below emitter voltage through a base resistor. Current flows into the emitter and out of the collector. If this current is too small (or zero), the base is starved and collector current shuts off. When you pull enough current out of the base, the emitter-base acts like a diode. The way you know you are pulling enough current out of the base is by checking the voltage between emitter and base. When transistor is on full, the base will be several hundred millivolts below the emitter voltage (< 500 mV if barely on, usually 600-900 mV when full on).

Bipolar Smoke Warning: You must limit the curent in bipolar devices. Two ways to do that. (1) Limit Ib. Use a resistor feeding into the base to limit Ib. Since Ib controls Ic, that will limit Ic. (typical for NPN switching) (2) Limit Ic. Use a resistor in series with the collector to limit Ic (emitter follower circuit). If you don't limit one or the other, you will get smoke.

If you're not sure which to do, USE A BASE RESISTOR OR BOTH. In a switch, Ib can get large, even if Ic is limited.


BIPOLAR SWITCHES vs AMPLIFIER: NPNs and PNPs can be used as switches, buffers or amplifiers. When used as a switch (the easiest), you're cranking the device hard-on or hard-off, nothing in between. Using a transistor as an amplifier requires significantly more design finesse to get linear gain and proper behavior.

NPN SWITCH DESCRIPTION: Emitter to ground. Resistor (e.g. 10k) between collector and Vcc. Resistor (e.g. 3k) between input voltage and base. Output = collector voltage. When input goes high (5V, 12V, whatever), transistor turns on, pulls current through collector resistor, and output goes low. When input goes to zero, no current can go into the NPN base. Transistor shuts off, collector voltage pulled to Vcc. When Ic is on fully, voltage between collector and emitter may be around 0.2V. Normal NPN Switching Operation for hobbiests: Emitter tied to ground for switching. Squirt current into base to turn on. Base will be 600-900 mV above the emitter when on. If collector drops below base voltage (saturation in switch operation), the base will start sucking in more current.


PNP SWITCH DESCRIPTION: Emitter to Vcc. Resistor (e.g. 10k) between collector and Ground. Resistor (e.g. 3k) between input voltage and base. Output = collector voltage. When input goes to ground, transistor turns on, pushes current through collector resistor, and output goes high. When input goes to Vcc, no current can flow out of the PNP base. Transistor shuts off, collector voltage pulled to Ground. When Ic is on fully, voltage between collector and emitter may be around 0.2V. Normal PNP operation for hobbiests: Emitter tied to Vcc for switching. Pull current out of base to turn on. Base will be 600-900 mV below the emitter when on. If collector rises above base voltage (saturation in switch operation), the base will start sending out more current.

(Bart McCoy, bartomccoy@gmail.com, 3/8/2008)

Links:

FET, MOSFET, JFET

CMOS / MOSFET

MOSFET = Metal-Oxide Semiconductor Field Effect Transistor

CMOS = Complementary Metal-Oxide Semiconductor

These are voltage controlled devices. The current is controlled by difference between the gate voltage (Vg) and source voltages (Vs). When the difference is large enough (above a threshold voltage called Vt (listed in datasheets), the transistor turns on.

TERMINALS: Drain, Gate, Source

NFETS AND PFETS: Like Bipolar devices, there are two types of FET devices as well. They are built differently.

N-Channel FETS (NFETS): NFETS turn on by raising the Gate voltage (Vg) above the Source voltage (Vs). This difference is called Vgs. When Vgs is more than some threshold voltage (Vt), the device turns on. In other words, when Vg is Vt or more volts above Vs, it turns on. Current flows INTO the drain and out of the source. The basic relationship is Id = Constant * (Vgs-Vt)^2. In switching applications or LED control applications, always tie the NFET source to ground. Apply TTL voltage directly to the gate. A resistor is tied between the drain and Vdd. If an LED is used, then the resistor controls the LED current and the LED is placed in series with the resistor.

P-Channel FETS (PFETS): PFETS turn on by lowering the Gate voltage (Vg) below the Source voltage (Vs). This difference is called Vsg. When Vsg is more than some threshold voltage (Vt), the device turns on. In other words, when Vg is Vt or more volts below Vs, it turns on. Current flows INTO the source and OUT OF the drain. The basic relationship is Id = Constant * (Vsg-Vt)^2. In switching applications or LED control applications, always tie the PFET source to Vdd. Apply TTL voltage directly to the gate. A resistor is tied between the drain and Ground. If an LED is used, then the resistor controls the LED current and the LED is placed in series with the resistor.

Hints Raise a NFET gate up to turn it on. Pull a PFET gate down to turn it on.

IMPORTANT NOTE ABOUT MOSTFET GATES: Unlike Bipolar devices, there is ZERO current going in/out of the gate (gate is very high impedance). That means you MUST force the gate voltage where you want it. If left to float, it will move about randomly, turning the transistor on and off unpredictably. If you have a situation where the gate is floating, tie the gate to Ground or Vdd through a large pullup resistor (e.g. 10-20K) to pin down the voltage to a default value of your choice.


ANALOGY BETWEEN MOSFETS AND BIPOLAR TRANSISTORS

Although Bipolar and FET devices are TOTALLY different, they have "black box" similarities. It is helpful to think about the similaries in how both technologies are used, but they are only for drawing an analogy, not to compare technical operation:

Terminals Description: Both are 3-Terminal Devices.

High impedance node: Collector/Drain

Control node: Base/Gate

Control Reference Node: Emitter/Source


Turn-On / Turn-Off:

Bipolar: Turned on by small base currents flowing in/out of base. A voltage between base and emitter (Vbe) will arise from this current flow. Ic = Ib * Hfe

FET: Turned on by pure voltage (Vgs). Id = Constant * Vgs^2


Current-Voltage Relationship:

Bipolar: Current turns on sharply with respect to Vbe, exponential-law

FET: Softer turn on with respect to Vgs (square-law)


N and P devices:

NPN: On when current flowing into base, one diode drop (Vbe) forms between base and emitter. Tie emitter to ground when using as a switch.

NFET: Device on when Vgs gets above Vt. Tie source to ground when using as a switch.


PNP: On when current pulled out of base, one diode drop (Vbe) forms between base and emitter. Tie emitter to Vcc when using as a switch.

PFET: Device on when Vsg gets above Vt. Tie source to Vdd when using as a switch.

Other Reading

(Original text Bart McCoy, bartomccoy(at)gmail.com, 3/8/2008)

Transistors Transdiode

This is a connection of a transistor to use it as a diode.


Further reading

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