This page documents all of the basic circuit sections that are included on my blog.
Voltage Divider
The first and most simple circuit is the voltage divider, which can simply be calculated from the ratio of resistor values. The formula used to calculate the output voltage is:
Vout = Vin(R2/(R1 + R2))
An alternative circuit can be seen on the right which uses a zener diode to keep the output at a specific voltage when the input is high enough, any additional voltage is dissipated across R1.
Transistor Buffer
A useful circuit that I commonly use is the 'emitter follower' circuit, which uses a transistor and a resistor to provide a very high resistance repeater between circuit sections.
Vin must always be 0.7v higher than Vout for this circuit to work, this is called its 'breakdown voltage'. Above this voltage the transistor is fully on and will exhibit current gain equal to its respective gain factor, which is usually around one hundred. This factor also controls the resistance seen at Vin and is equal to R1 multiplied by its gain factor.
Operational Amplifiers
The op-amp is a very useful component and you will see it a lot in modern circuit diagrams, it is a very versatile component with many uses, some examples are given below.
Comparator
Another name given to the op-amp usually dictated by context of use within a circuit, the comparator as its name suggests compares two voltage inputs and amplifies the difference by a very large number.
The output voltage is quite discrete, it is either equal to the supply voltage if the positive input is higher than the negative input or ground (0v) if vice versa.
Buffer
The simplest op-amp circuit is the buffer, the output is dictated by the input going into the positive terminal and negative feedback is used to stabilize the output.
Inverting Amplifier
Similar to the concept used in the buffer, the inverting amplifier uses negative feedback but using a ratio of resistors to control the output amplification; equal to -R2/R1.
For AC applications a DC offset can be set so there is no signal clipping, this is usually set to half of the supply voltage.
Relaxation Oscillators
The 555 timer is a classic square-wave oscillator circuit, the frequency of which is roughly equal to:
1/((R2)(C1)).
R1 is normally 1k and does not really change. It is used to control the hysteresis and duty cycle of the circuit.
The product of R2 and C1 is the exponential time constant and controls the rise and fall of the voltage that controls the on/off triggering, hence the 555-timer can also be used as a sawtooth wave generator.
Similar to the 555 timer as the schmitt trigger uses the same equation to work out frequency but only uses one resistor as hysteresis is intrinsic to the design of the component.
There are 6 schmitt triggers on a single chip so this circuit is very useful when a few oscillators are needed in the same place.
RLC Filter Circuit Basics
The depth that could be shown on RLC type circuits is quite extensive and can involve the use of complex numbers and Laplace transforms in the frequency domain. This section only exists to give brief and simplified explanation of the implementation of RLC filter circuits.RC Filter
The RC filter is composed of a resistor and a capacitor and is considered a first order system because it only has one frequency domain component (the capacitor in this case). Don't sweat the details.
This circuit can be viewed as a voltage divider for the sake of a simple explanation and the capacitor's "resistance" normally called capacitive reactance can be quantified using the formula below.
Where Xc is the reactance, F is the frequency of the signal and C is the capacitance.
So for a DC signal the reactance will be infinite (open circuit).
But for AC signals you can quantify it using the frequency of the AC signal.
Note: that in this arrangement you will see an increase in attenuation of the circuit as the input frequency increases making it a low pass filter. To change it to a high pass simply swap the components around.
You can use the frequency dependency of this circuit with the formula provided to find out how the voltage of the AC signal will change with frequency and calculate what component values you need in order to filter the undesired frequencies of the signal.
RL Filter
The RL filter is composed of a resistor and an inductor and again, is considered a first order system because it only has one frequency domain component (the capacitor in this case). Don't sweat the details.
As before this circuit can be viewed as a voltage divider for the sake of a simple explanation and the inductor's "resistance" normally called inductive reactance can be quantified using the formula below.
Where Xl is the reactance, F is the frequency of the signal and L is the inductance.
So for a DC signal the reactance will be zero (closed circuit).
But for AC signals you can quantify it using the frequency of the AC signal.
Note: that in this arrangement you will see an increase in attenuation of the circuit as the input frequency increases making it a low pass filter. To change it to a high pass simply swap the components around.
LC Filter
The LC filter is composed of two reactive components and so is slightly different from the resistor based system. It is considered to be a second order system because of the two frequency dependent components.
This circuit can also be viewed as a voltage divider and the reluctances act as pseudo-resistors, they both must be taken into account when figuring out the attenuation of the input signal.
Where L is the inductance and C is the capacitance.