A voltage amplifier in simplest form is any circuit that puts out a higher voltage than the input voltage. When you are forced to work with a set amount of voltage, these amplifiers are commonly used to increase the voltage and thus the amount of power coming out of a circuit. This is useful for reading and adapting small signals such as boosting an audio signal before sending it on its way to speakers. The voltage amplifier is a form of the common emitter amplifier, which relies on the transistor; the amplification of voltage is dependent on the ratio of resistors on the collector and emitter of this transistor.
The following materials are for an amplifier with a gain of 10. If you want to increase or decrease this factor, refer to step 2.
To build this circuit, you will need the materials listed below. Names of specific instruments used in this particular circuit are included in parentheses.
- function generator (BK Precision 4011A 5MHz Function Generator)
- breadboard (Global Specialties Proto-Board PB-503)
- DC power supply (15V, included in our breadboard)
- transistor (Q1 2N3904)
- capacitor - 100nF
- resistors - 56 kOhm, 5.6 kOhm, 6.8 kOhm, 680 Ohm
If you are using this circuit for practical purposes, you can use any DC power supply you desire; keep in mind that your output voltage can not be larger than the voltage provided by this DC power supply. Therefore I would recommend power supplies in the range of 9 to 15V DC. The sine input from the function generator is simply the input that you wish to be amplified.
Additionally, you will likely want something to read or use the output voltage produced by this circuit, depending on your reasons for wanting to build a voltage amplifier. If you are simply looking to investigate the circuit, an oscilloscope can be used to read the output voltage.
Again, this circuit has an voltage gain of 10. For different values of gain, different resistors will be needed (see step 2).
Step 1: Building the Circuit
The first image is the schematic of the circuit; the bottom right is the breadboard view; and the bottom left is our final product.
Let's talk a little about the circuit before you start building it. If we completely ignore the sine input, we can see that there will still be a complete circuit and therefore a voltage at the output. Based on the ratios of the resistors used here, this voltage (called the quiescent voltage) calculates out to be about half of the input voltage. With the voltage already at such a large quantity, it makes sense that the addition of this sine input voltage, even if small, will lead to a much larger output voltage. Hence the amplifier!
A note about the capacitor:
The first two resistors are what set the DC voltage of the base. The capacitor acts to filter out the DC current from the function generator, leaving only a sine wave so as to keep the AC voltage from interfering with the DC voltages.
So now lets begin building the thing! The most efficient and wire free way to go about doing this would be to leave all connectors until the end. Starting with the transistor and the resistors, the circuit looks easy enough so I wont try to guide you through it. Once this base is done, the function generator (bottom left in red and black) can be connected and then output (which is shown here as the purple wire connected to a probe).
If you are going to look at the results on an oscilloscope, I recommend using the TTL output of the function generator as the external trigger for the scope. If you are actually going to hook it up to sometime, I still stand for checking your work first-- you never know what can go wrong!