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!

<p>I'm building this amplifier to drive a loudspeaker hooked to a signal generator. This will probably sound like a dumb question, but would this amplifier work with square wave output from the generator?</p>
<p>Great website!</p><p>I just built this circuit, connected to a DC supply and a function generator. Unfortunately, it's doing the opposite: 5V in and 1V out. What could be wrong? I saw no effect of the DC supply (the same result with and without the DC supply) :-(</p><p>Thanks!</p>
<p>I just built this circuit to amplify a control voltage signal ( for a synthesizer) from a reference of 0V to 0.58V, and it was totally successful. However, that input CV voltage changes depending on the note (i.e., voltage) being sent. What this circuit does not do is change output voltage relative to input voltage. The output stays at ~0.58V no matter what the voltage in. Ideally, the voltage out would maintain that 0.58V increase for any input voltage (e.g., 1V = 1.58V, 2.36V = 2.92V, etc.) Any suggestions of the type of circuit that would do this, or if this circuit can be modified to do so? </p>
<p>A decoupled 0.58V source in series.</p>
<p>Question -- I am using a signal generator where the input voltage is 3.6 or 5v and the subsequent output frequency has around .5 volts. However, in the end I am looking to have around 35-volts. Initially I was planning on using a voltage booster circuit but it seems getting from .5-volts to 35-volts is a big hurdle. I was going to put the signal booster before the signal generator (AD9850 chip based) but that only allows for up to a 5-volt input I believe. Any thoughts??? Also, when the current goes through the voltage booster does it change the frequency at all? I am not concered with the loss of amperage since I am looking at needing only microamps (in the 20-600 microamp range).</p>
<p>Connect a transistor like this: B to resistor to your 0-0.5V output, C to resistor to 35V, E to GND, then you have 0-35V on C.</p>
<p>so what would i need to double the voltage?(what parts)</p>
<p>how do i amplify a 0-0.5v signal to 0-3.3v?</p>
<p>Hi, I would like to amplify a 100-200 mv pulse to 5v DC. The frequency of the pulse would vary between 20-600 per second. At lover frequency the pulse is 100-200 mv, So I would like to amplify it to 5 volt. But at higher frequency the pulse goes upto 30-50 volts. So I would like to maintain the pulse to 5 volts at all frequencies So I need the schematics for it.</p>
<p>like this: alt+m=&micro;. then, just add cap. f. :)</p>
<p>15 mhz --- then switch to grounded base</p>
<p>the emitter resistance is .025 / Ic ?? seems to make sense --- Va = Rc/Re = 5000/25 = 200 --- Carl Stancil is on facebook</p>
Dear! if i replace this transistor with 2SC5200, what will be the maximum frequency that i can get from this circuit. your reply will be very helpful for me.
Will this be a good enough bump up in power to drive a Flyback transformer? <br>
Could you modify this to show the effect of adding an emitter bypass capacitor as well ?
Nice electronics 101 'Ible.

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