Biasing A Transistor Amplifier

Pictured above is a DC Biasing circuit for an NPN transistor used as an amplifier. Oscium’s iMSO-104 handheld oscilloscope measures the input and output voltage swings as the DC bias point, measured and displayed on the Fluke DMM, is varied to obtain a symmetrical output. The DC source is a simple AC/DC converter that has been modified slightly to be compatible with the prototyping board seen in the picture. The AC signal for the circuit is generated by one of Agilent’s Arbitrary Waveform Generators.

Transistor Biasing Basics (NPN)

Simple DC Biasing Circuit

This circuit is simple to assemble and troubleshoot. There are fewer components used in this circuit than more elaborate biasing schemes require. The individual components can be swapped and replaced one at a time to see the effects on the performance of the circuit. In this way you can collect enough information to get intuition about how the circuit functions and how to tailor it so meet the specific needs of your application. For my measurements, I used a potentiometer between VCC and Ground to set the Base-to-Emitter voltage.

One drawback to this circuit is that the quiescent current through the collector and emitter is highly dependent on the Beta value of the individual transistor. Inexpensive transistors may have more Beta variation, and therefore this circuit would have limited control over the quiescent current and resulting operating point.

Collector Current: Family Of Curves

The figure below shows the family of curves for current through the transistor’s collector as the collector-emitter voltage increases, for different base current bias points. This plot appears in almost every transistor datasheet because it helps users predict and understand the performance of the device under different operating conditions.

Combine Small Signal With DC Bias

The sketch below shows a simple circuit for applying a DC voltage and small signal to the base of a transistor. The DC voltage sets the transistor’s operating point and the small signal is amplified at the collector of the transistor.

Physical Implementation Of The NPN Transistor Amplifier

The components used to implement the circuit all came from my neighborhood RadioShack. An essential component not pictured below is the DC supply. I used an Enercell AC/DC power converter with a selectable output voltage for VCC. The potentiometer allows me to vary the voltage applied to the transistor’s base while VCC remains constant. The active device in the circuit is an NPN transistor in a power package with flange for optional heat sink.

Collector Current vs Base-Emitter Voltage

Pictured below is the current plot for a forward biased transistor or a forward biased diode. The two devices have a lot in common and therefore they have similar bias current plots. Once the transistor is turned “ON”, where the base-emitter voltage is greater than threshold, the current through the transistor increases exponentially.

Transistor Regions Of Operation

The graphic below depicts the three regions of operation for bipolar transistors. For amplifier applications, the transistor needs to be in the Active region, biased along one of the base current curves shown. When used as a switch, however, transistors are used in the Cutoff and Saturation regions.

Transistor Operating Point

The last piece to designing a transistor amplifier circuit is choosing the operating point. Output swing, quiescent current, efficiency, and distortion and linearity are the performance parameters most important for consideration. The sketch below depicts an operating point (big blue dot) at the intersection of the two collector current curves plotted. The next step would be to collect data on these parameters at different operating points for comparison and analysis.