Amplifier DC parameter test (1)

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Amplifier DC parameter test (1) [Copy link]

Amplifier DC parameter test (1)

Hi, uu, what are you busy with recently? Have you decided where to go for May 1 and Dragon Boat Festival? If you want to see the mountains and the sea, you can come to my hometown, Cangnan, Wenzhou.

Let's get straight to the point. There are many DC parameters of amplifiers. In bench testing, automated testing is required. Usually, many relays are needed to switch different configurations to measure different parameters. Here, Renesas provides a test reference circuit. See Figure 1.

Figure 1: DC key parameter test circuit

The switching mode of Relay is shown in Table 1:

Parameter Measurements		Switch Positions[1]
		S1	S2	S3	S4	S5
DC(Output =TP1)	Vos	1	1	0	A	a
	lB-	0	1	0	A	a
	IB+	1	0	0	A	a
	AOL	1	1	1	A	a
	CMRR	1	1	0	B	b
	PSRR	1	1	0	C	c

Offset voltage (Vos)

The input offset voltage of an ideal amplifier is 0, but in reality, this is not the case. In the absence of any input, there is still an offset voltage at the input. The model of the offset voltage is shown in Figure 2 below.

Figure 2: Offset voltage model

Note that the offset voltage is not necessarily positive or negative. It is described in the EC Table of the amplifier manual. Now, back to the Bench test circuit of Renesas, the configuration for measuring Vos can be simplified as shown in Figure 3 below.

Figure 3: Offset test configuration

Many amplifiers in simulation software do not have offset voltage, so we use V3 to set an input offset voltage. Let's see, at this time, X1's IN+ is greater than IN-, so should X1's output be high? If X1's output is high, then let's look at X2. X2 is an integrator. When X1's output is higher than GND, X2's output is low. When X2's output makes X1's IN- lower than 3mV, the situation is reversed. So we can derive X1's offset based on this principle. X2's output voltage is equal to the Offset voltage when divided by Gain. This offset voltage will add an X2 offset, but X2's offset will not be amplified, so there is more room for choice. Let's look at the simulation waveform, as shown in Figure 4 below.

Figure 4: Vos simulation waveform

Input bias current (IB)

The ideal amplifier has infinite input impedance and does not require input bias current, but this is not the case with actual amplifiers. The input bias current model of the amplifier is shown in Figure 5.

Figure 5: Amplifier input bias current model

In the input structure of a bipolar amplifier, two current sources are usually used as bias for IN+ and IN-, but they cannot be completely matched, which will cause Ios. However, before that, we need to measure the current. According to the settings in Table 1, we modify the circuit as shown in Figure 6 below to measure IB-.

Figure 6: IB-measurement circuit

Since we have the basis for the first measurement of X2-out, we can directly look at the output value of X2-out after switching the Relay. A simple simulation is performed to see the output result, as shown in Figure 7. The simple calculation result is shown in Figure 8, which is consistent with the simulation result.

Figure 7: IB simulation results

Figure 8: IB-calculation results

CalcPad:

'First output voltage' VOUT1 = 3.0108883V 'Second output voltage' VOUT2 = 3.0066828V 'Output voltage difference' VDIF = VOUT1 - VOUT2|mV 'Voltage gain' VGain = 1000 'Input resistance change' R_in = 100kΩ 'Final IB' iB = (VDIF/VGain)/R_in|pA

It is basically consistent, but some amplifiers have very small input bias current, and the output change measured with this architecture is very small. Then we can try another method. The book on basic linear circuit design gives a test method, as shown in Figure 9.

Figure 9: Ultra-low bias current measurement

The simplified circuit is shown in Figure 10. In the real world, capacitors may have various leakage or temperature drifts, so when using this circuit, you must pay attention to the selection of capacitors. Use polytetrafluoroethylene or polypropylene type capacitors. Figure 9 has already explained the calculation method of IB. Let's just look at the output simulation waveform. As shown in Figure 11.

Figure 10: Ultra-low bias current measurement

Figure 11: Simulation results

We see that from 0S to 10S, it just rises to 4.2V. According to the calculation formula, we get the following conclusion. The calculation process is shown in Figure 12.

Figure 12: Calculation process

CalcPad:

'Input capacitance' c = 100pF '△Vout voltage' deltaVout = 4.2V '△Time' deltaT = 10s 'Bias current' iB = c*(deltaVout/deltaT)|pA

Open-Loop Gain Measurement (AOL)

The circuit for measuring open-loop Gain is similar to that for measuring Vos, but a bias voltage is added to the integrator. According to the configuration in Table 1, we change the simulation drawing as shown in Figure 13.

Figure 13: AOL test circuit

If you want to keep the input voltage of X2 in- at 0V, then you must make X1 output -5V. At this point we can draw a conclusion. Only when the input of X2-out changes by 1000:1, the output of X1_Out changes to 5V, then its Gain=1000*(1V/X2_out). The final simulation results are shown in Figure 14. The calculation results are shown in Figure 15.

Figure 14: AOL simulation results

Figure 15: Open-loop gain calculation

CalcPad:

Vout1 = 3.0108883V Vout2 = 3.0327471V 'Voltage difference' Vdif = Vout2 - Vout1|mV 'Gain' Gain = 1000*(5V/Vdif) 'Convert to dB' AOL = 20* log (Gain)

That’s all for today!

About the author: Xu Tong, 8.5 years of work experience, circuit system architecture expert, 14 years of experience in the circuit field, proficient in application circuit system architecture design, more than 10 circuit architecture patents, mastered multiple circuit design skills, circuit Spice simulation, C language, Python, Verilog, etc.

Reference Documentation

Bench-Testing Important DC-Parameters of Operational Amplifiers Renesas

Basic Linear Design Analog Devices

Simple Op Amp Measurements Analog Devices

qwqwqw2088

When measuring ultra-low bias current, there are leakage or temperature drift issues. Can the software detect these issues?

Then in the actual circuit, the capacitor is a polytetrafluoroethylene or polypropylene type

qwqwqw2088

Or should we use PTFE or polypropylene capacitors directly in the actual circuit according to experience to avoid such problems?

How to use simulation to identify and discover these leakage or temperature drift problems

xutong

qwqwqw2088 posted on 2024-4-22 21:32 Or should we use the actual circuit directly to avoid such problems with polytetrafluoroethylene or polypropylene type capacitors according to the empirical value? How to use simulation to determine and discover these leakage...

A complete capacitor model can be constructed and may be downloaded from the capacitor manufacturer's website.

大智若愚123

When measuring ultra-low bias current, there are leakage or temperature drift issues. Can the software detect these issues?

xutong

大智若愚123 Published on 2024-8-5 12:29 Is there any leakage or temperature drift problem when measuring ultra-low bias current? Can the software detect these problems?

It depends on your model.