Very-high-gain non-inverting digital amplifier/switch using npn transistors When the input is “high,” Q1 is driven to saturation and pulls Q2 base to less than 600mV, so Q2 is cut off and the output is high (at V+).įIGURE 3.
When the input is at zero volts, Q1 is cut off, so Q2 is driven fully on via R2, and the output is low (saturated). The Figure 3 circuit uses two npn transistors. Alternatively, a very-high-gain non-inverting digital amplifier/switch can be made by using a pair of transistors wired in either of the ways shown in Figures 3 or 4. The sensitivity of the Figure 1 and 2 circuits can be increased by replacing Q1 with a pair of Darlington- or Super-Alpha-connected transistors. Q1 switches fully on, with its output a few hundred mV below the positive supply value, when the input is at zero volts, and turns off (with its output at zero volts) when the input rises to within less than 600mV of the positive supply rail value. In practice, R b should be as small as possible, consistent with safety and input-impedance requirements, and must not exceed R L x h fe.įigure 2 shows a pnp version of the digital inverter/switch circuit. This snag can be overcome by shunting R b with a “speed-up” capacitor (typically about 1n0), as shown dotted in the diagram. The circuit’s input impedance is slightly greater than the R b value, which also influences the rise and fall times of the output signal - the greater the R b value, the worse these become. In Figure 1, resistor R b limits the input base-drive current to a safe value. Thus, the output signal is an amplified and inverted version of the input signal. If the input voltage is large enough, Q1 is driven fully on and the output drops to a “saturation” value of a few hundred mV. When the input is high, the transistor is biased on and a collector current flows via R L, thus pulling the output low.
#3 RESISTOR NPN TRANSISTOR AMPLIFIER FULL#
When the input is zero, the transistor is cut off and the output is at full positive supply rail value.
#3 RESISTOR NPN TRANSISTOR AMPLIFIER SERIES#
DIGITAL CIRCUITSįigure 1 shows a simple npn common-emitter digital amplifier, inverter, or switch, in which the input signal is at either zero volts or a substantial positive value, and is applied to the transistor’s base via series resistor R b, and the output signal is taken from the transistor’s collector.
This section of the Cookbook series starts off by looking at “digital” application circuits. The common-emitter amplifier can be used in a wide variety of digital and analog voltage amplifier applications.
The circuit’s input is applied to the transistor’s base, and the output is taken from its collector - the circuit’s basic operating principles were briefly described in the opening installment of this eight-part series. The common-emitter amplifier (also known as the common-earth or grounded-emitter circuit) has a medium value of input impedance and provides substantial voltage gain between input and output. This article moves on and shows various ways of using bipolar transistors in simple, but useful common-emitter and common-base configurations. Last time in this series, I described practical ways of using bipolar transistors in useful common-collector (voltage follower) circuit applications, including those of relay drivers, constant-current generators, linear amplifiers, and complementary emitter followers.
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