Explore Working Applications of 8 Different Op-Amp Circuits - DC To Daylight — Transcript

Full transcript

1. 0:00Hi, I'm Derek, and this is DC to Daylight. In this  episode, we're going to take a look at a handful
2. 0:05of OP-AMP circuits. Eight, to be exact. Now in  the previous episode we looked at the inverting,
3. 0:09non-inverting end comparator, we also looked  under the hood in an OP-AMP. If you want a
4. 0:14little more of a detailed explanation of what's  going on, click the link above and you can get
5. 0:18to that previous episode. So this video is really  just going to be a cursory overview of all these
6. 0:22circuits, and the basic formulas. How to use  them, and what values in the neighborhood you
7. 0:27should be using to make the OP-AMP circuit do what  it needs to do. No proofs (or) fancy derivations,
8. 0:33we'll leave that to the textbook. This is  more of a practical application approach.
9. 0:37Just showing you how to use the thing, so  we're going to look at the inverting amplifier,
10. 0:41the non-inverting amplifier, a comparator peak  detector summing amplifier, difference amplifier,
11. 0:48and we'll throw in for good measure:  the clipper and clamper circuit. So
12. 0:52let's not spend any more time talking  about it! Let's get right into it.
13. 1:01So the first circuit on our list is the inverting  amplifier, in this amplifier we have basically two
14. 1:06resistors. We have a feedback resistor that forms  our closed loop system, we're feeding the output
15. 1:11back to the non-inverting input, and we have  an input resistance that goes to the inverting
16. 1:16terminal. The non-inverting terminal is connected  directly to ground. Now in this particular circuit
17. 1:21the ratio of the feedback resistor, to the  input resistor, is proportional to the voltages
18. 1:27involved. So the gain of the system is negative,  because, it's an inverting amp the output is
19. 1:33180 degrees out of phase with the input. So the  feedback resistor divided by the input resistance
20. 1:38is equal to your gain. So let's breadboard that  circuit and we'll take a look at it on the scope.
21. 1:43So for all experiments today we're going to  be using the TL072 from Texas Instruments,
22. 1:48uh, much better performance than the LM741, and  has a low noise JFET. For an end, I've already set
23. 1:54up my power supply here, I have positive 12 going  to, uh, pin 8. I have a negative 12 volts going to
24. 2:00the negative supply here on pin 4 and you'll see  the bare wires are connected to ground, which is
25. 2:05halfway between that supply. Some OP-AMPS need a  dual supply, some only require a single supply. In
26. 2:11this case I'm using a dual, just for simplicity's  sake. Okay so I have bare wires here that are
27. 2:16ground, and I have decoupling capacitors going  from the positive rail to ground, and the negative
28. 2:21rail to ground, you should actually use these  at all times because they shunt noise to ground,
29. 2:27and it's often omitted from schematics. so pin  1 is the output, pin 2 is the inverting input,
30. 2:32and pin 3 is non-inverting. So the non-inverting  input in this case is connected to ground,
31. 2:37I'm going to use a 10k resistor for my  inverting input to my signal source, here,
32. 2:43coming from my function generator I'm going to use  a feedback resistor of 47 kilo ohms. So my gain is
33. 2:494.7 if I have a half a volt in, 0.5 times 4.7, my  output voltage should be 2.35 volts. So over at
34. 2:59the scope you can see I have my half a volt peak  to peak signal here, and when I turn the OP-AMP on
35. 3:06I should see 2.35v and in fact we're measuring  2.38v, and you'll notice that the phase is 180
36. 3:14degrees out of phase at the input. So the  blue trace is my output, yellow is my input,
37. 3:17and they are flipped 180 degrees out of  phase so that is the inverting OP-AMP.
38. 3:23The second circuit we're going to look at today  is the non-inverting amplifier. Now as the name
39. 3:28implies, the phase of the output is in phase  with the input signal, there is no inversion,
40. 3:32so we drive the non-inverting pin directly  with our signal, and at the output we have
41. 3:37a voltage divider. Two resistors and we  tap off the center of those resistors,
42. 3:40and feed it back into the inverting input. Now  what's unique about the non-inverting amplifier,
43. 3:45is that the gain is of course the ratio of r2  / 1. Just like the previous one, but there's
44. 3:51no phase inversion, so there's no negative sign.  But, we do have to add plus one, and that's the
45. 3:56way the math works out. I'm not getting into  the math like i said, but, it's 1 + r2 / r1,
46. 4:02that 1 means that the gain can never be less  than 1. Whereas the inverting amplifier we
47. 4:08could actually use it as an attenuator. All that  being said, let's go over to the breadboard, and
48. 4:14we'll actually measure it. Now let's reconfigure  this OP-AMP as a non-inverting circuit, we're
49. 4:19going to use our 47k as our feedback resistor  which goes from pin 1 to 2, and remember,
50. 4:25we're forming a voltage divider. So from pin 2  I'm going to use a 10k resistor, going to ground
51. 4:31which we do there, now we're going to feed the  input at pin 3. Okay, remember the gain is the
52. 4:37ratio of those two resistors, but we have to add  one because we're using this voltage divider here.
53. 4:42Recall that, uh, 47 kilo-ohms divided by our 10  kilo-ohms is equal to 4.7 + 1. Our gain is 5.7,
54. 4:50and we're going to multiply that by the  500 millivolts that we're inputting. So
55. 4:54times 0.5 we should see 2.85 volts peak-to-peak  on the scope. So we see that we are in phase,
56. 5:02the output has the same phasing as the input and  down here, we actually measure 2.9v. So yeah, our
57. 5:10scope isn't 100% accurate. I trust a volt meter  better than this, but you can see that we have
58. 5:14a gain of 2.85v close to 3v. The third circuit  we're going to look at today is the comparator,
59. 5:20and as the name implies we're comparing two  voltages at the inputs of the OP-AMP. Now we
60. 5:24need a reference voltage to compare it to, so in  our case we're going to use a voltage divider,
61. 5:29and we're going to connect it to the inverting  pin of the OP-AMP. Now, you could use any kind
62. 5:34of semiconductor specialized device to set up  a voltage reference, or you could use a diode,
63. 5:38a zener diode, something to create a stable  voltage, to compare our input signal against.
64. 5:43Now when our input voltage gets above our  reference voltage, the output simply slams high.
65. 5:48Okay, when our input voltage is less than  a reference voltage, the output slams low.
66. 5:52Pretty simple, let's see that thing in action on  the bench! All right, so this is our comparator
67. 5:57circuit. So I have a voltage divider that acts  as a reference, going to our inverting pin, and
68. 6:02attached to our non-inverting pin is the center  wiper of this potentiometer. So as I turn this,
69. 6:09I can change the voltage on the non-inverting  pin, the OP-AMP is going to compare that varying
70. 6:15voltage against our fixed voltage here, which is  only a couple of volts below our negative rail. So
71. 6:19we'll look at it on the scope and you'll see as I  vary this potentiometer, I'm changing the voltage
72. 6:23and it'll kind of pass over and under that  reference voltage, so the output pin will be
73. 6:28monitoring as well, and I have it connected to  a couple of LEDs. So as I'm above my reference
74. 6:34voltage will go green if I'm below it, will  turn red, okay, because these LEDs are flipped
75. 6:39in opposite polarities. Let me turn this on, and  we'll monitor our potentiometer using this probe,
76. 6:49and we'll monitor the output of the OP-AMP  using this probe. So over at the scope I
77. 6:55have my reference voltage, okay, which is just,  uh, like a volt or so above the negative rail,
78. 7:01and then I've got my blue trace. So as I vary  my potentiometer I can go over and under that,
79. 7:07right, so the OP-AMP is going to compare these two  and give us a certain output. So back over on our
80. 7:12breadboard, if I'm above that reference voltage  on green and as I decrease my potentiometer
81. 7:19as I cross over that threshold, it turns  red. Well, it looks orange on camera,
82. 7:23I'm going to go back above, and it turns  green. Now with this probe right here,
83. 7:26we're going to monitor the output of the  OP-AMP, so let me turn that trace on.
84. 7:31All right, you can see that we're 'high',  because that LED was green. Now if I go below
85. 7:36my threshold voltage the purple trace will go low.  All right, so that's what the comparator is doing,
86. 7:44so the fourth circuit that we're going to  look at today is called the peak detector,
87. 7:48and essentially it's used in audio  circuits, and anytime you want to
88. 7:52grab a signal and hold it high, for some period of  time, okay, like driving a VU meter for some kind
89. 7:57of audio circuitry. Right, the way this thing  works is we just drive the OP-AMP with some
90. 8:01kind of signal and at the output of the OP-AMP we  put a diode, and that acts as a rectifier. Okay,
91. 8:06so the output of that rectifier gets fed  back into the other terminal of the OP-AMP
92. 8:11so that forms a precision rectifier. Now  if we connect a capacitor to ground, okay,
93. 8:16across the output shunted to ground, what we do  is when that signal goes high, it charges up that
94. 8:21capacitor rather quickly and then it discharges.  So if we're connected to a meter it can hold that
95. 8:26signal high for a period of time, and then we  have that natural discharge curve of a RC circuit,
96. 8:31which brings us to another point if you have  a resistor connected across the output of that
97. 8:34OP-AMP, it will drain faster. So you really have  to pay attention to your RC time constant. Okay,
98. 8:39anyway, let's take it to the bench. You can see  exactly what I'm talking about all right, so the
99. 8:43peak detector circuit is actually pretty simple.  Okay, we have a diode that goes from the output
100. 8:48and goes into the non-inverting input.  The signal that we want to peak detect,
101. 8:52okay, our input signal goes to pin 3 which is our  non-inverting input. I've got a probe here that's
102. 8:57measuring the output of that diode on our scope,  our yellow trace is our input signal and the blue
103. 9:02trace is the output waveform from that rectifier  diode. So yeah, we've got some undershoot here
104. 9:09where the diode's turning on and off but what  we're going to do, we're going to take a 680
105. 9:13picofarad capacitor and we're going to connect  it between the output of that diode and ground,
106. 9:18and what that's going to do is we'll attempt  to smooth out that rectified waveform, right,
107. 9:22just like a regular rectifier circuit, and you  can immediately see on the scope that we start
108. 9:28to fill in the gaps here. Okay that capacitor  gets charged up, and then starts discharging
109. 9:33and that discharge rate is an RC time constant.  So if you want to fill in this gap more and hold
110. 9:39that peak longer, then what you need to do is  increase that value of capacitance. So let's
111. 9:44change that value of 680 picofarads, and we'll  change it to 10 nanofarads, pull this guy out,
112. 9:51we'll put this guy in, and there you go! Now you  can see what the peak detector is really doing!
113. 9:56So we're charging up, we're holding, charging  up, holding, so when I turn that off,
114. 10:02and you can see we have kind of, this sine wave,  that starts to form, it holds the peak there we
115. 10:07charge it up again, it holds it again, so  if I were listening to music, there might
116. 10:10be a big bass response, and then it would hold  it for a little bit and discharge. All right,
117. 10:15and then the next base peak would come up and  we would charge up again, so you've got to know
118. 10:19what kind of load you're driving and how the time  constant is going to affect that discharge rate,
119. 10:23okay. So that is how a peak detector works the  fifth circuit we're going to look at today is
120. 10:28the summing amplifier sometimes called a mixer  or adder. All right so in this case if we're
121. 10:34using a non-inverting amplifier we have multiple  resistors at the input and one feedback resistor,
122. 10:39okay. So again the ratio of the feedback resistor  to the input resistors sets the gain of the
123. 10:45system, okay, so, typically we'll have the same  resistance across all of those input resistors
124. 10:50and the ratio of, uh, r2, or r, feedback over  whatever that input resistance might be sets the
125. 10:56gain of the system, so we could take one of those  signals, it could be a sine wave the other signal
126. 11:01could be i don't know plus five volts and then  at the output you'd have that sine wave shifted
127. 11:05up by five volts okay so you can use it to apply  an offset you can also mix audio signals in fact
128. 11:10those big mixing boards that you see typically end  up going through one of these summing amplifiers
129. 11:16okay let's take a look okay here's our  summing amplifier it's basically an inverting
130. 11:20configuration okay so my input resistors are  both going to pin 2 which is my inverting input
131. 11:26and i have my feedback resistor here going from  the output back to the inverting input just like
132. 11:32any inverting amplifier my non-inverting terminal  is connected to ground all of these resistors are
133. 11:375.6 k so overall the gain okay the closed loop  gain is 1. so whatever i see at the input i
134. 11:43should see at the output but i'm adding these  two terminals together so nothing connected to
135. 11:49this terminal but i have a potentiometer connected  to the other input terminal so let's connect our
136. 11:53probe to the output of the op amp and see what we  get so as i turn this potentiometer you can see
137. 11:58that my output voltage from the op amp changes  so what happens if i add a sine wave to this
138. 12:04well let's connect a sine wave to the other  resistor and you can see we have a sine wave
139. 12:11at the output on our scope let me turn on the  reference waveform that's actually driving that op
140. 12:16amp input and that's in yellow okay so my function  generator driving the op amp is yellow the output
141. 12:23from the op-amp is 180 degrees out of phase as  i turn the potentiometer okay i go more positive
142. 12:30or an inverting amplifier so it drives the sine  wave down we're adding the dc offset from the pot
143. 12:36to the sine wave that we're inputting on the  other pin okay now i could add another channel
144. 12:41three four five other inputs to this op-amp and  we could come up with a really wacky waveform
145. 12:46but i just wanted to show you how we can add  two signals together and there you go summing
146. 12:51amplifier the sixth circuit that we're going to  look at today is called a difference amplifier
147. 12:56so any time that we have a balanced signal uh  maybe a long cable run to some sensor it can pick
148. 13:01up a lot of noise all right so from rf anything in  the environment like a drill or a motor the noise
149. 13:07will be on that line in phase okay on both of  those signals now we'll have a differential signal
150. 13:13along that line that we want to keep and the  difference amplifier if you have common mode noise
151. 13:17on each line and you subtract those signals the  noise essentially goes away and that's dependent
152. 13:22upon the cmrr or common mode rejection ratio of  the op amp all right and that's in your data sheet
153. 13:28the higher the number in dbs the better it is  at getting rid of that noise all right and your
154. 13:33differential signal will still be there all right  and this is a pretty cool one so let's take a look
155. 13:38here's our differential amplifier circuit i've  got two input resistors that are connected to
156. 13:43our inverting input and our non-inverting  input and they each have their own feedback
157. 13:48resistors so we have basically a non-inverting  and inverting circuit with feedback resistors
158. 13:54that work in concert to subtract the  difference between these two input signals
159. 13:59all the values are the same at 5.6 k so it gives  us a gain of 1. now i could have made these
160. 14:06feedback resistors equal and we could introduce  gain but i'm trying to keep things simple here
161. 14:11what i'm going to do is introduce some  noise through these capacitors on ac couple
162. 14:17random noise and that is going to be common to  both inputs okay so we're driving both of these
163. 14:24pins at the same time with the same noise of the  same phase and it's connected to each of the input
164. 14:28side of these resistors and i'm running out of  scope probes so we're just going to connect this
165. 14:33guy up like this this guy goes to ground all right  so let's take a look at that noise there it is on
166. 14:39the screen with the purple trace it's a 3.4 volts  peak to peak and that noise on the other side of
167. 14:44the capacitor is going to be superimposed on that  sine wave for each of these input legs of the
168. 14:51op amp now remember my noise is in phase and these  two signals are out of phase okay so we're going
169. 14:57to subtract all of that and what we're going  to end up with at the output is hopefully just
170. 15:03the sine wave so it has subtracted out the noise  and this blows me away every time that i see it
171. 15:09it's absolutely amazing that that differential  amplifier can subtract out that noise and just
172. 15:13leave the sine wave the seventh circuit that we're  going to look at today is called a clipper so the
173. 15:19clipper is an interesting circuit that well in my  experience it comes from guitar okay so distortion
174. 15:25pedals it takes your nice clean input from your  guitar and it chops off the waveform and makes
175. 15:31it sound crunchy and you know heavy metal the  other application for clipper circuits is maybe
176. 15:38taking a sine wave and squaring it off so that  you can measure the frequency or maybe clipping
177. 15:42a circuit before it goes into i don't know an  arduino so that you're not damaging the input pins
178. 15:47as a esd protection device something like  that so anyway let's look at how that works
179. 15:52all right so in this case we're driving uh our  input resistor with our sine wave on this side
180. 15:57and on the other side of this input resistor  we're driving the non-inverting terminal this
181. 16:01diode connects to the output of the op amp uh  the cathode side anyway the anode side goes to
182. 16:05the non-inverting input along with this resistor  uninverting terminal is connected to a voltage
183. 16:10reference which is our potentiometer okay so  as we change this potentiometer we change the
184. 16:15reference voltage and where that signal clips and  we're monitoring the output at the anode side of
185. 16:22the diode not at the output of the op-amp but on  this side of the diode okay let's see what that
186. 16:27waveform looks like here's our input waveform  in yellow let me turn on the output trace okay
187. 16:32right on top of that signal and as i adjust my  potentiometer we change where our clipping occurs
188. 16:42so neat little circuit now if i wanted to clip  the top side and the bottom side of that sine
189. 16:46wave i could cascade a couple of op-amps  and flip the polarity of this diode and
190. 16:51essentially you'd be able to control both sides  with a voltage reference like i'm doing here
191. 16:56so there you go the last circuit we're going to  look at today is called a clamper now the clamper
192. 17:00is an interesting circuit that doesn't get a lot  of love basically if you have an ac input signal
193. 17:04and you want to apply some offset instead of  adding a dc voltage you can just throw a capacitor
194. 17:09in the circuit and that will provide the dc offset  for you so anyway interesting circuit let's take a
195. 17:15look on the breadboard alright so here we have our  clamper circuit it's very similar in configuration
196. 17:19to the clipper circuit except we have a capacitor  that we're driving all right our input signal is
197. 17:24driving one leg of the cap and the output is being  measured across the other side of that cap we also
198. 17:28have a diode but in this case the polarity is  reversed so the anode is connected to the output
199. 17:33cathode to the inverting input the non-inverting  input is grounded all right so what this thing
200. 17:39does is it'll actually lift your input signal up  above ground okay so it doesn't really provide
201. 17:46a variable offset it lifts that signal to where  the bottom part of that uh let's say a sine wave
202. 17:51rides along ground and everything is shifted up  interesting little circuit so let's uh look at
203. 17:57the scope and we'll turn the op amp on and you'll  see we have our zero volt reference here and that
204. 18:02sine wave is riding right along top of it okay  that's basically how a clamper works alright well
205. 18:09that wraps up this episode of op amp applications  my hope with this episode was that by showing you
206. 18:15these different circuits and configurations that  it would give you the confidence to build your own
207. 18:20OP-AMP circuits if you run across like a sensor or  something you need to interface to another device
208. 18:25so it puts it in your mental toolbox and you can  use this as a reference so hopefully it was useful
209. 18:30if it was let me know down in the comments  or hit me up at community.element14.com in
210. 18:34the community the links are down below  and that is it i will see you next time!